Configuration Parameters

Descriptions of the LightDB-A Database server configuration parameters listed alphabetically.

application_name

Sets the application name for a client session. For example, if connecting via psql, this will be set to psql. Setting an application name allows it to be reported in log messages and statistics views.

array_nulls

This controls whether the array input parser recognizes unquoted NULL as specifying a null array element. By default, this is on, allowing array values containing null values to be entered. LightDB-A Database versions before 3.0 did not support null values in arrays, and therefore would treat NULL as specifying a normal array element with the string value ‘NULL’.

authentication_timeout

Maximum time to complete client authentication. This prevents hung clients from occupying a connection indefinitely.

autovacuum

When enabled, LightDB-A Database starts up the autovacuum daemon, which operates at the database level. After the daemon is running, all databases have their catalog tables automatically vacuumed and their catalog and user tables automatically analyzed.

backslash_quote

This controls whether a quote mark can be represented by \’ in a string literal. The preferred, SQL-standard way to represent a quote mark is by doubling it (“) but PostgreSQL has historically also accepted \’. However, use of \’ creates security risks because in some client character set encodings, there are multibyte characters in which the last byte is numerically equivalent to ASCII .

block_size

Reports the size of a disk block.

bonjour_name

Specifies the Bonjour broadcast name. By default, the computer name is used, specified as an empty string. This option is ignored if the server was not compiled with Bonjour support.

check_function_bodies

When set to off, deactivates validation of the function body string during CREATE FUNCTION. Deactivating validation is occasionally useful to avoid problems such as forward references when restoring function definitions from a dump.

client_connection_check_interval

Sets the time interval between optional checks that the client is still connected, while running queries. 0 deactivates connection checks.

client_encoding

Sets the client-side encoding (character set). The default is to use the same as the database encoding. See Supported Character Sets in the PostgreSQL documentation.

client_min_messages

Controls which message levels are sent to the client. Each level includes all the levels that follow it. The later the level, the fewer messages are sent.

INFO level messages are always sent to the client.

cpu_index_tuple_cost

For the Postgres Planner, sets the estimate of the cost of processing each index row during an index scan. This is measured as a fraction of the cost of a sequential page fetch.

cpu_operator_cost

For the Postgres Planner, sets the estimate of the cost of processing each operator in a WHERE clause. This is measured as a fraction of the cost of a sequential page fetch.

cpu_tuple_cost

For the Postgres Planner, Sets the estimate of the cost of processing each row during a query. This is measured as a fraction of the cost of a sequential page fetch.

cursor_tuple_fraction

Tells the Postgres Planner how many rows are expected to be fetched in a cursor query, thereby allowing the Postgres Planner to use this information to optimize the query plan. The default of 1 means all rows will be fetched.

data_checksums

Reports whether checksums are enabled for heap data storage in the database system. Checksums for heap data are enabled or deactivated when the database system is initialized and cannot be changed.

Heap data pages store heap tables, catalog tables, indexes, and database metadata. Append-optimized storage has built-in checksum support that is unrelated to this parameter.

LightDB-A Database uses checksums to prevent loading data corrupted in the file system into memory managed by database processes. When heap data checksums are enabled, LightDB-A Database computes and stores checksums on heap data pages when they are written to disk. When a page is retrieved from disk, the checksum is verified. If the verification fails, an error is generated and the page is not permitted to load into managed memory.

If the ignore_checksum_failure configuration parameter has been set to on, a failed checksum verification generates a warning, but the page is allowed to be loaded into managed memory. If the page is then updated, it is flushed to disk and replicated to the mirror. This can cause data corruption to propagate to the mirror and prevent a complete recovery. Because of the potential for data loss, the ignore_checksum_failure parameter should only be enabled when needed to recover data. See ignore_checksum_failure for more information.

DateStyle

Sets the display format for date and time values, as well as the rules for interpreting ambiguous date input values. This variable contains two independent components: the output format specification and the input/output specification for year/month/day ordering.

db_user_namespace

This enables per-database user names. If on, you should create users as username@dbname. To create ordinary global users, simply append @ when specifying the user name in the client.

deadlock_timeout

The time to wait on a lock before checking to see if there is a deadlock condition. On a heavily loaded server you might want to raise this value. Ideally the setting should exceed your typical transaction time, so as to improve the odds that a lock will be released before the waiter decides to check for deadlock.

debug_assertions

Turns on various assertion checks.

debug_pretty_print

Indents debug output to produce a more readable but much longer output format. client_min_messages or log_min_messages must be DEBUG1 or lower.

debug_print_parse

For each query run, prints the resulting parse tree. client_min_messages or log_min_messages must be DEBUG1 or lower.

debug_print_plan

For each query run, prints the LightDB-A parallel query execution plan. client_min_messages or log_min_messages must be DEBUG1 or lower.

debug_print_prelim_plan

For each query run, prints the preliminary query plan. client_min_messages or log_min_messages must be DEBUG1 or lower.

debug_print_rewritten

For each query run, prints the query rewriter output. client_min_messages or log_min_messages must be DEBUG1 or lower.

debug_print_slice_table

For each query run, prints the LightDB-A query slice plan. client_min_messages or log_min_messages must be DEBUG1 or lower.

default_statistics_target

Sets the default statistics sampling target (the number of values that are stored in the list of common values) for table columns that have not had a column-specific target set via ALTER TABLE SET STATISTICS. Larger values may improve the quality of the Postgres Planner estimates, particularly for columns with irregular data distributions, at the expense of consuming more space in pg_statistic and slightly more time to compute the estimates. Conversely, a lower limit might be sufficient for columns with simple data distributions.

default_table_access_method

Sets the default table access method when a CREATE TABLE command does not explicitly specify an access method.

default_tablespace

The default tablespace in which to create objects (tables and indexes) when a CREATE command does not explicitly specify a tablespace.

default_text_search_config

Selects the text search configuration that is used by those variants of the text search functions that do not have an explicit argument specifying the configuration. See Using Full Text Search for further information. The built-in default is pg_catalog.simple, but initdb will initialize the configuration file with a setting that corresponds to the chosen lc_ctype locale, if a configuration matching that locale can be identified.

default_transaction_deferrable

When running at the SERIALIZABLE isolation level, a deferrable read-only SQL transaction may be delayed before it is allowed to proceed. However, once it begins running it does not incur any of the overhead required to ensure serializability; so serialization code will have no reason to force it to abort because of concurrent updates, making this option suitable for long-running read-only transactions.

This parameter controls the default deferrable status of each new transaction. It currently has no effect on read-write transactions or those operating at isolation levels lower than SERIALIZABLE. The default is off.

Note Setting default_transaction_deferrable to on has no effect in LightDB-A Database. Only read-only, SERIALIZABLE transactions can be deferred. However, LightDB-A Database does not support the SERIALIZABLE transaction isolation level. See SET TRANSACTION.

default_transaction_isolation

Controls the default isolation level of each new transaction. LightDB-A Database treats read uncommitted the same as read committed, and treats serializable the same as repeatable read.

default_transaction_read_only

Controls the default read-only status of each new transaction. A read-only SQL transaction cannot alter non-temporary tables.

dynamic_library_path

If a dynamically loadable module needs to be opened and the file name specified in the CREATE FUNCTION or LOAD command does not have a directory component (i.e. the name does not contain a slash), the system will search this path for the required file. The compiled-in PostgreSQL package library directory is substituted for $libdir. This is where the modules provided by the standard PostgreSQL distribution are installed.

effective_cache_size

Sets the assumption about the effective size of the disk cache that is available to a single query for the Postgres Planner. This is factored into estimates of the cost of using an index; a higher value makes it more likely index scans will be used, a lower value makes it more likely sequential scans will be used. When setting this parameter, you should consider both LightDB-A Database’s shared buffers and the portion of the kernel’s disk cache that will be used for data files (though some data might exist in both places). Take also into account the expected number of concurrent queries on different tables, since they will have to share the available space. This parameter has no effect on the size of shared memory allocated by a LightDB-A server instance, nor does it reserve kernel disk cache; it is used only for estimation purposes.

Set this parameter to a number of block_size blocks (default 32K) with no units; for example, 262144 for 8GB. You can also directly specify the size of the effective cache; for example, '1GB' specifies a size of 32768 blocks. The gpconfig utility and SHOW command display the effective cache size value in units such as 'GB’, 'MB’, or 'kB’.

enable_bitmapscan

Activates or deactivates the use of bitmap-scan plan types by the Postgres Planner. Note that this is different than a Bitmap Index Scan. A Bitmap Scan means that indexes will be dynamically converted to bitmaps in memory when appropriate, giving faster index performance on complex queries against very large tables. It is used when there are multiple predicates on different indexed columns. Each bitmap per column can be compared to create a final list of selected tuples.

enable_groupagg

Activates or deactivates the use of group aggregation plan types by the Postgres Planner.

enable_hashagg

Activates or deactivates the use of hash aggregation plan types by the Postgres Planner.

enable_hashjoin

Activates or deactivates the use of hash-join plan types by the Postgres Planner.

enable_indexscan

Activates or deactivates the use of index-scan plan types by the Postgres Planner.

enable_mergejoin

Activates or deactivates the use of merge-join plan types by the Postgres Planner. Merge join is based on the idea of sorting the left- and right-hand tables into order and then scanning them in parallel. So, both data types must be capable of being fully ordered, and the join operator must be one that can only succeed for pairs of values that fall at the 'same place’ in the sort order. In practice this means that the join operator must behave like equality.

enable_nestloop

Activates or deactivates the use of nested-loop join plans by the Postgres Planner. It’s not possible to suppress nested-loop joins entirely, but turning this variable off discourages the Postgres Planner from using one if there are other methods available.

enable_partition_pruning

Enables or disables the query planner’s ability to eliminate a partitioned table’s partitions from query plans. This also controls the planner’s ability to generate query plans which allow the query executor to remove (ignore) partitions during query execution.

enable_seqscan

Activates or deactivates the use of sequential scan plan types by the Postgres Planner. It’s not possible to suppress sequential scans entirely, but turning this variable off discourages the Postgres Planner from using one if there are other methods available.

enable_sort

Activates or deactivates the use of explicit sort steps by the Postgres Planner. It’s not possible to suppress explicit sorts entirely, but turning this variable off discourages the Postgres Planner from using one if there are other methods available.

enable_tidscan

Activates or deactivates the use of tuple identifier (TID) scan plan types by the Postgres Planner.

escape_string_warning

When on, a warning is issued if a backslash (\) appears in an ordinary string literal (’…’ syntax). Escape string syntax (E’…’) should be used for escapes, because in future versions, ordinary strings will have the SQL standard-conforming behavior of treating backslashes literally.

explain_pretty_print

Determines whether EXPLAIN VERBOSE uses the indented or non-indented format for displaying detailed query-tree dumps.

extra_float_digits

Adjusts the number of digits displayed for floating-point values, including float4, float8, and geometric data types. The parameter value is added to the standard number of digits. The value can be set as high as 3, to include partially-significant digits; this is especially useful for dumping float data that needs to be restored exactly. Or it can be set negative to suppress unwanted digits.

from_collapse_limit

The Postgres Planner will merge sub-queries into upper queries if the resulting FROM list would have no more than this many items. Smaller values reduce planning time but may yield inferior query plans.

gp_adjust_selectivity_for_outerjoins

Enables the selectivity of NULL tests over outer joins.

gp_appendonly_compaction

Enables compacting segment files during VACUUM commands. When deactivated, VACUUM only truncates the segment files to the EOF value, as is the current behavior. The administrator may want to deactivate compaction in high I/O load situations or low space situations.

gp_appendonly_compaction_threshold

Specifies the threshold ratio (as a percentage) of hidden rows to total rows that triggers compaction of the segment file when VACUUM is run without the FULL option (a lazy vacuum). If the ratio of hidden rows in a segment file on a segment is less than this threshold, the segment file is not compacted, and a log message is issued.

gp_autostats_allow_nonowner

The gp_autostats_allow_nonowner server configuration parameter determines whether or not to allow LightDB-A Database to trigger automatic statistics collection when a table is modified by a non-owner.

The default value is false; LightDB-A Database does not trigger automatic statistics collection on a table that is updated by a non-owner.

When set to true, LightDB-A Database will also trigger automatic statistics collection on a table when:

• gp_autostats_mode=on_change and the table is modified by a non-owner.
• gp_autostats_mode=on_no_stats and the first user to INSERT or COPY into the table is a non-owner.

The gp_autostats_allow_nonowner configuration parameter can be changed only by a superuser.

gp_autostats_mode

Specifies the mode for triggering automatic statistics collection with ANALYZE. The on_no_stats option triggers statistics collection for CREATE TABLE AS SELECT, INSERT, or COPY operations on any table that has no existing statistics.

The on_change option triggers statistics collection only when the number of rows affected exceeds the threshold defined by gp_autostats_on_change_threshold. Operations that can trigger automatic statistics collection with on_change are:

CREATE TABLE AS SELECT

UPDATE

DELETE

INSERT

COPY

Default is on_no_stats.

Note For partitioned tables, automatic statistics collection is not triggered if data is inserted from the top-level parent table of a partitioned table.

Automatic statistics collection is triggered if data is inserted directly in a leaf table (where the data is stored) of the partitioned table. Statistics are collected only on the leaf table.

gp_autostats_mode_in_functions

Specifies the mode for triggering automatic statistics collection with ANALYZE for statements in procedural language functions. The none option deactivates statistics collection. The on_no_stats option triggers statistics collection for CREATE TABLE AS SELECT, INSERT, or COPY operations that are run in functions on any table that has no existing statistics.

The on_change option triggers statistics collection only when the number of rows affected exceeds the threshold defined by gp_autostats_on_change_threshold. Operations in functions that can trigger automatic statistics collection with on_change are:

CREATE TABLE AS SELECT

UPDATE

DELETE

INSERT

COPY

gp_autostats_on_change_threshold

Specifies the threshold for automatic statistics collection when gp_autostats_mode is set to on_change. When a triggering table operation affects a number of rows exceeding this threshold, ANALYZE is added and statistics are collected for the table.

gp_cached_segworkers_threshold

When a user starts a session with LightDB-A Database and issues a query, the system creates groups or 'gangs’ of worker processes on each segment to do the work. After the work is done, the segment worker processes are destroyed except for a cached number which is set by this parameter. A lower setting conserves system resources on the segment hosts, but a higher setting may improve performance for power-users that want to issue many complex queries in a row.

gp_command_count

Shows how many commands the coordinator has received from the client. Note that a single SQLcommand might actually involve more than one command internally, so the counter may increment by more than one for a single query. This counter also is shared by all of the segment processes working on the command.

gp_connection_send_timeout

Timeout for sending data to unresponsive LightDB-A Database user clients during query processing. A value of 0 deactivates the timeout, LightDB-A Database waits indefinitely for a client. When the timeout is reached, the query is cancelled with this message:

Could not send data to client: Connection timed out.

gp_content

The local content id if a segment.

gp_create_table_random_default_distribution

Controls table creation when a LightDB-A Database table is created with a CREATE TABLE or CREATE TABLE AS command that does not contain a DISTRIBUTED BY clause.

For CREATE TABLE, if the value of the parameter is off (the default), and the table creation command does not contain a DISTRIBUTED BY clause, LightDB-A Database chooses the table distribution key based on the command:

• If a LIKE or INHERITS clause is specified, then LightDB-A copies the distribution key from the source or parent table.
• If a PRIMARY KEY or UNIQUE constraints are specified, then LightDB-A chooses the largest subset of all the key columns as the distribution key.
• If neither constraints nor a LIKE or INHERITS clause is specified, then LightDB-A chooses the first suitable column as the distribution key. (Columns with geometric or user-defined data types are not eligible as LightDB-A distribution key columns.)

If the value of the parameter is set to on, LightDB-A Database follows these rules to create a table when the DISTRIBUTED BY clause is not specified:

• If PRIMARY KEY or UNIQUE columns are not specified, the distribution of the table is random (DISTRIBUTED RANDOMLY). Table distribution is random even if the table creation command contains the LIKE or INHERITS clause.
• If PRIMARY KEY or UNIQUE columns are specified, a DISTRIBUTED BY clause must also be specified. If a DISTRIBUTED BY clause is not specified as part of the table creation command, the command fails.

For a CREATE TABLE AS command that does not contain a distribution clause:

• If the Postgres Planner creates the table, and the value of the parameter is off, the table distribution policy is determined based on the command.
• If the Postgres Planner creates the table, and the value of the parameter is on, the table distribution policy is random.
• If GPORCA creates the table, the table distribution policy is random. The parameter value has no affect.

For information about the Postgres Planner and GPORCA, see "Querying Data” in the LightDB-A Database Administrator Guide.

gp_dbid

The local content dbid of a segment.

gp_debug_linger

Number of seconds for a LightDB-A process to linger after a fatal internal error.

gp_default_storage_options

Set the default values for the following table storage options when a table is created with the CREATE TABLE command.

• appendoptimized

  Note You use the appendoptimized=value syntax to specify the append-optimized table storage type. appendoptimized is a thin alias for the appendonly legacy storage option. LightDB-A Database stores appendonly in the catalog, and displays the same when listing the storage options for append-optimized tables.

• blocksize

• checksum

• compresstype

• compresslevel

• orientation

Specify multiple storage option values as a comma separated list.

You can set the storage options with this parameter instead of specifying the table storage options in the WITH of the CREATE TABLE command. The table storage options that are specified with the CREATE TABLE command override the values specified by this parameter.

Not all combinations of storage option values are valid. If the specified storage options are not valid, an error is returned. See the CREATE TABLE command for information about table storage options.

The defaults can be set for a database and user. If the server configuration parameter is set at different levels, this the order of precedence, from highest to lowest, of the table storage values when a user logs into a database and creates a table:

1. The values specified in a CREATE TABLE command with the WITH clause or ENCODING clause
2. The value of gp_default_storage_options that set for the user with the ALTER ROLE...SET command
3. The value of gp_default_storage_options that is set for the database with the ALTER DATABASE...SET command
4. The value of gp_default_storage_options that is set for the LightDB-A Database system with the gpconfig utility

The parameter value is not cumulative. For example, if the parameter specifies the appendoptimized and compresstype options for a database and a user logs in and sets the parameter to specify the value for the orientation option, the appendoptimized, and compresstype values set at the database level are ignored.

This example ALTER DATABASE command sets the default orientation and compresstype table storage options for the database mytest.

ALTER DATABASE mytest SET gp_default_storage_options = 'orientation=column, compresstype=rle_type'

To create an append-optimized table in the mytest database with column-oriented table and RLE compression. The user needs to specify only appendoptimized=TRUE in the WITH clause.

This example gpconfig utility command sets the default storage option for a LightDB-A Database system. If you set the defaults for multiple table storage options, the value must be enclosed in single quotes.

gpconfig -c 'gp_default_storage_options' -v 'appendoptimized=true, orientation=column'

This example gpconfig utility command shows the value of the parameter. The parameter value must be consistent across the LightDB-A Database coordinator and all segments.

gpconfig -s 'gp_default_storage_options'

Note ¹The set classification when the parameter is set at the system level with the gpconfig utility.

Note ²QuickLZ compression is available only in the commercial release of VMware LightDB-A.

gp_dispatch_keepalives_count

Maximum number of TCP keepalive retransmits from a LightDB-A Query Dispatcher to its Query Executors. It controls the number of consecutive keepalive retransmits that can be lost before a connection between a Query Dispatcher and a Query Executor is considered dead.

gp_dispatch_keepalives_idle

Time in seconds between issuing TCP keepalives from a LightDB-A Query Dispatcher to its Query Executors.

gp_dispatch_keepalives_interval

Time in seconds between TCP keepalive retransmits from a LightDB-A Query Dispatcher to its Query Executors.

gp_dynamic_partition_pruning

Enables plans that can dynamically eliminate the scanning of partitions.

gp_eager_two_phase_agg

Activates or deactivates two-phase aggregation for the Postgres Planner.

The default value is off; the Planner chooses the best aggregate path for a query based on the cost. When set to on, the Planner adds a disable cost to each of the first stage aggregate paths, which in turn forces the Planner to generate and choose a multi-stage aggregate path.

gp_enable_agg_distinct

Activates or deactivates two-phase aggregation to compute a single distinct-qualified aggregate. This applies only to subqueries that include a single distinct-qualified aggregate function.

gp_enable_agg_distinct_pruning

Activates or deactivates three-phase aggregation and join to compute distinct-qualified aggregates. This applies only to subqueries that include one or more distinct-qualified aggregate functions.

gp_enable_direct_dispatch

Activates or deactivates the dispatching of targeted query plans for queries that access data on a single segment. When on, queries that target rows on a single segment will only have their query plan dispatched to that segment (rather than to all segments). This significantly reduces the response time of qualifying queries as there is no interconnect setup involved. Direct dispatch does require more CPU utilization on the coordinator.

gp_enable_fast_sri

When set to on, the Postgres Planner plans single row inserts so that they are sent directly to the correct segment instance (no motion operation required). This significantly improves performance of single-row-insert statements.

gp_enable_global_deadlock_detector

Controls whether the LightDB-A Database Global Deadlock Detector is enabled to manage concurrent UPDATE and DELETE operations on heap tables to improve performance. See Inserting, Updating, and Deleting Datain the LightDB-A Database Administrator Guide. The default is off, the Global Deadlock Detector is deactivated.

If the Global Deadlock Detector is deactivated (the default), LightDB-A Database runs concurrent update and delete operations on a heap table serially.

If the Global Deadlock Detector is enabled, concurrent updates are permitted and the Global Deadlock Detector determines when a deadlock exists, and breaks the deadlock by cancelling one or more backend processes associated with the youngest transaction(s) involved.

gp_enable_groupext_distinct_gather

Activates or deactivates gathering data to a single node to compute distinct-qualified aggregates on grouping extension queries. When this parameter and gp_enable_groupext_distinct_pruning are both enabled, the Postgres Planner uses the cheaper plan.

gp_enable_groupext_distinct_pruning

Activates or deactivates three-phase aggregation and join to compute distinct-qualified aggregates on grouping extension queries. Usually, enabling this parameter generates a cheaper query plan that the Postgres Planner will use in preference to existing plan.

gp_enable_multiphase_agg

Activates or deactivates the use of two or three-stage parallel aggregation plans Postgres Planner. This approach applies to any subquery with aggregation. If gp_enable_multiphase_agg is off, thengp_enable_agg_distinct and gp_enable_agg_distinct_pruning are deactivated.

gp_enable_predicate_propagation

When enabled, the Postgres Planner applies query predicates to both table expressions in cases where the tables are joined on their distribution key column(s). Filtering both tables prior to doing the join (when possible) is more efficient.

gp_enable_preunique

Enables two-phase duplicate removal for SELECT DISTINCT queries (not SELECT COUNT(DISTINCT)). When enabled, it adds an extra SORT DISTINCT set of plan nodes before motioning. In cases where the distinct operation greatly reduces the number of rows, this extra SORT DISTINCT is much cheaper than the cost of sending the rows across the Interconnect.

gp_enable_query_metrics

Enables collection of query metrics. When query metrics collection is enabled, LightDB-A Database collects metrics during query execution. The default is off.

After changing this configuration parameter, LightDB-A Database must be restarted for the change to take effect.

The LightDB-A Database metrics collection extension, when enabled, sends the collected metrics over UDP to a VMware LightDB-A Command Center agent¹.

Note ¹The metrics collection extension is included in VMware’s commercial version of LightDB-A Database. VMware LightDB-A Command Center is supported only with VMware LightDB-A.

gp_enable_relsize_collection

Enables GPORCA and the Postgres Planner to use the estimated size of a table (pg_relation_size function) if there are no statistics for the table. By default, GPORCA and the planner use a default value to estimate the number of rows if statistics are not available. The default behavior improves query optimization time and reduces resource queue usage in heavy workloads, but can lead to suboptimal plans.

This parameter is ignored for a root partition of a partitioned table. When GPORCA is enabled and the root partition does not have statistics, GPORCA always uses the default value. You can use ANALZYE ROOTPARTITION to collect statistics on the root partition. See ANALYZE.

gp_enable_segment_copy_checking

Controls whether the distribution policy for a table (from the table DISTRIBUTED clause) is checked when data is copied into the table with the COPY FROM...ON SEGMENT command. If true, an error is returned if a row of data violates the distribution policy for a segment instance. The default is true.

If the value is false, the distribution policy is not checked. The data added to the table might violate the table distribution policy for the segment instance. Manual redistribution of table data might be required. See the ALTER TABLE clause WITH REORGANIZE.

The parameter can be set for a database system or a session. The parameter cannot be set for a specific database.

gp_enable_sort_limit

Enable LIMIT operation to be performed while sorting. Sorts more efficiently when the plan requires the first limit_number of rows at most.

gp_external_enable_exec

Activates or deactivates the use of external tables that run OS commands or scripts on the segment hosts (CREATE EXTERNAL TABLE EXECUTE syntax). Must be enabled if using the Command Center or MapReduce features.

gp_external_max_segs

Sets the number of segments that will scan external table data during an external table operation, the purpose being not to overload the system with scanning data and take away resources from other concurrent operations. This only applies to external tables that use the gpfdist:// protocol to access external table data.

gp_external_enable_filter_pushdown

Enable filter pushdown when reading data from external tables. If pushdown fails, a query is run without pushing filters to the external data source (instead, LightDB-A Database applies the same constraints to the result). See Defining External Tables for more information.

gp_fts_probe_interval

Specifies the polling interval for the fault detection process (ftsprobe). The ftsprobe process will take approximately this amount of time to detect a segment failure.

gp_fts_probe_retries

Specifies the number of times the fault detection process (ftsprobe) attempts to connect to a segment before reporting segment failure.

gp_fts_probe_timeout

Specifies the allowed timeout for the fault detection process (ftsprobe) to establish a connection to a segment before declaring it down.

gp_fts_replication_attempt_count

Specifies the maximum number of times that LightDB-A Database attempts to establish a primary-mirror replication connection. When this count is exceeded, the fault detection process (ftsprobe) stops retrying and marks the mirror down.

gp_global_deadlock_detector_period

Specifies the executing interval (in seconds) of the global deadlock detector background worker process.

gp_log_endpoints

Controls the amount of parallel retrieve cursor endpoint detail that LightDB-A Database writes to the server log file.

The default value is false, LightDB-A Database does not log endpoint details to the log file. When set to true, LightDB-A writes endpoint detail information to the log file.

gp_log_fts

Controls the amount of detail the fault detection process (ftsprobe) writes to the log file.

gp_log_interconnect

Controls the amount of information that is written to the log file about communication between LightDB-A Database segment instance worker processes. The default value is terse. The log information is written to both the coordinator and segment instance logs.

Increasing the amount of logging could affect performance and increase disk space usage.

gp_log_gang

Controls the amount of information that is written to the log file about query worker process creation and query management. The default value is OFF, do not log information.

gp_hashjoin_tuples_per_bucket

Sets the target density of the hash table used by HashJoin operations. A smaller value will tend to produce larger hash tables, which can increase join performance.

gp_ignore_error_table

Controls LightDB-A Database behavior when the deprecated INTO ERROR TABLE clause is specified in a CREATE EXTERNAL TABLE or COPY command.

Note The INTO ERROR TABLE clause was deprecated and removed in LightDB-A Database 5. In LightDB-A Database 7, this parameter will be removed as well, causing all INTO ERROR TABLE invocations be yield a syntax error.

The default value is false, LightDB-A Database returns an error if the INTO ERROR TABLE clause is specified in a command.

If the value is true, LightDB-A Database ignores the clause, issues a warning, and runs the command without the INTO ERROR TABLE clause. In LightDB-A Database 5.x and later, you access the error log information with built-in SQL functions. See the CREATE EXTERNAL TABLE or COPY command.

You can set this value to true to avoid the LightDB-A Database error when you run applications that run CREATE EXTERNAL TABLE or COPY commands that include the LightDB-A Database 4.3.x INTO ERROR TABLE clause.

gp_initial_bad_row_limit

For the parameter value n, LightDB-A Database stops processing input rows when you import data with the COPY command or from an external table if the first n rows processed contain formatting errors. If a valid row is processed within the first n rows, LightDB-A Database continues processing input rows.

Setting the value to 0 deactivates this limit.

The SEGMENT REJECT LIMIT clause can also be specified for the COPY command or the external table definition to limit the number of rejected rows.

INT_MAX is the largest value that can be stored as an integer on your system.

gp_instrument_shmem_size

The amount of shared memory, in kilobytes, allocated for query metrics. The default is 5120 and the maximum is 131072. At startup, if gp_enable_query_metrics is set to on, LightDB-A Database allocates space in shared memory to save query metrics. This memory is organized as a header and a list of slots. The number of slots needed depends on the number of concurrent queries and the number of execution plan nodes per query. The default value, 5120, is based on a LightDB-A Database system that runs a maximum of about 250 concurrent queries with 120 nodes per query. If the gp_enable_query_metrics configuration parameter is off, or if the slots are exhausted, the metrics are maintained in local memory instead of in shared memory.

gp_interconnect_address_type

Specifies the type of address binding strategy LightDB-A Database uses for communication between segment host sockets. There are two types: unicast and wildcard. The default is wildcard.

• When this parameter is set to unicast, LightDB-A Database uses the gp_segment_configuration.address field to perform address binding. This reduces port usage on segment hosts and prevents interconnect traffic from being routed through unintended (and possibly slower) network interfaces.

• When this parameter is set to wildcard, LightDB-A Database uses a wildcard address for binding, enabling the use of any network interface compliant with routing rules.

Note In some cases, inter-segment communication using the unicast strategy may not be possible. One example is if the source segment’s address field and the destination segment’s address field are on different subnets and/or existing routing rules do not allow for such communication. In these cases, you must configure this parameter to use a wildcard address for address binding.

gp_interconnect_debug_retry_interval

Specifies the interval, in seconds, to log LightDB-A Database interconnect debugging messages when the server configuration parameter gp_log_interconnect is set to DEBUG. The default is 10 seconds.

The log messages contain information about the interconnect communication between LightDB-A Database segment instance worker processes. The information can be helpful when debugging network issues between segment instances.

gp_interconnect_fc_method

Specifies the flow control method used for the default LightDB-A Database UDPIFC interconnect.

For capacity based flow control, senders do not send packets when receivers do not have the capacity.

Loss based flow control is based on capacity based flow control, and also tunes the sending speed according to packet losses.

gp_interconnect_proxy_addresses

Sets the proxy ports that LightDB-A Database uses when the server configuration parameter gp_interconnect_type is set to proxy. Otherwise, this parameter is ignored. The default value is an empty string (“”).

When the gp_interconnect_type parameter is set to proxy, You must specify a proxy port for the coordinator, standby coordinator, and all primary and mirror segment instances in this format:

<db_id>:<cont_id>:<seg_address>:<port>[, ... ]

For the coordinator, standby coordinator, and segment instance, the first three fields, db_id, cont_id, and seg_address can be found in the gp_segment_configuration catalog table. The fourth field, port, is the proxy port for the LightDB-A coordinator or a segment instance.

• db_id is the dbid column in the catalog table.
• cont_id is the content column in the catalog table.
• seg_address is the IP address or hostname corresponding to tge address column in the catalog table.
• port is the TCP/IP port for the segment instance proxy that you specify.

Important If a segment instance hostname is bound to a different IP address at runtime, you must run gpstop -U to re-load the gp_interconnect_proxy_addresses value.

You must specify the value as a single-quoted string. This gpconfig command sets the value for gp_interconnect_proxy_addresses as a single-quoted string. The LightDB-A system consists of a coordinator and a single segment instance.

gpconfig --skipvalidation -c gp_interconnect_proxy_addresses -v "'1:-1:192.168.180.50:35432,2:0:192.168.180.54:35000'"

For an example of setting gp_interconnect_proxy_addresses, see Configuring Proxies for the LightDB-A Interconnect.

gp_interconnect_queue_depth

Sets the amount of data per-peer to be queued by the LightDB-A Database interconnect on receivers (when data is received but no space is available to receive it the data will be dropped, and the transmitter will need to resend it) for the default UDPIFC interconnect. Increasing the depth from its default value will cause the system to use more memory, but may increase performance. It is reasonable to set this value between 1 and 10. Queries with data skew potentially perform better with an increased queue depth. Increasing this may radically increase the amount of memory used by the system.

gp_interconnect_setup_timeout

Specifies the amount of time, in seconds, that LightDB-A Database waits for the interconnect to complete setup before it times out.

gp_interconnect_snd_queue_depth

Sets the amount of data per-peer to be queued by the default UDPIFC interconnect on senders. Increasing the depth from its default value will cause the system to use more memory, but may increase performance. Reasonable values for this parameter are between 1 and 4. Increasing the value might radically increase the amount of memory used by the system.

gp_interconnect_transmit_timeout

Specifies the amount of time, in seconds, that LightDB-A Database waits for network transmission of interconnect traffic to complete before it times out.

gp_interconnect_type

Sets the networking protocol used for LightDB-A Database interconnect traffic. UDPIFC specifies using UDP with flow control for interconnect traffic, and is the only value supported.

UDPIFC (the default) specifies using UDP with flow control for interconnect traffic. Specify the interconnect flow control method with gp_interconnect_fc_method.

With TCP as the interconnect protocol, LightDB-A Database has an upper limit of 1000 segment instances - less than that if the query workload involves complex, multi-slice queries.

The PROXY value specifies using the TCP protocol, and when running queries, using a proxy for LightDB-A interconnect communication between the coordinator instance and segment instances and between two segment instances. When this parameter is set to PROXY, you must specify the proxy ports for the coordinator and segment instances with the server configuration parameter gp_interconnect_proxy_addresses. For information about configuring and using proxies with the LightDB-A interconnect, see Configuring Proxies for the LightDB-A Interconnect.

gp_log_format

Specifies the format of the server log files. If using gp_toolkit administrative schema, the log files must be in CSV format.

gp_max_local_distributed_cache

Sets the maximum number of distributed transaction log entries to cache in the backend process memory of a segment instance.

The log entries contain information about the state of rows that are being accessed by an SQL statement. The information is used to determine which rows are visible to an SQL transaction when running multiple simultaneous SQL statements in an MVCC environment. Caching distributed transaction log entries locally improves transaction processing speed by improving performance of the row visibility determination process.

The default value is optimal for a wide variety of SQL processing environments.

gp_max_packet_size

Sets the tuple-serialization chunk size for the LightDB-A Database interconnect.

gp_max_parallel_cursors

Specifies the maximum number of active parallel retrieve cursors allowed on a LightDB-A Database cluster. A parallel retrieve cursor is considered active after it has been DECLAREd, but before it is CLOSEd or returns an error.

The default value is -1; there is no limit on the number of open parallel retrieve cursors that may be concurrently active in the cluster (up to the maximum value of 1024).

You must be a superuser to change the gp_max_parallel_cursors setting.

gp_max_plan_size

Specifies the total maximum uncompressed size of a query execution plan multiplied by the number of Motion operators (slices) in the plan. If the size of the query plan exceeds the value, the query is cancelled and an error is returned. A value of 0 means that the size of the plan is not monitored.

You can specify a value in kB, MB, or GB. The default unit is kB. For example, a value of 200 is 200kB. A value of 1GB is the same as 1024MB or 1048576kB.

gp_max_slices

Specifies the maximum number of slices (portions of a query plan that are run on segment instances) that can be generated by a query. If the query generates more than the specified number of slices, LightDB-A Database returns an error and does not run the query. The default value is 0, no maximum value.

Running a query that generates a large number of slices might affect LightDB-A Database performance. For example, a query that contains UNION or UNION ALL operators over several complex views can generate a large number of slices. You can run EXPLAIN ANALYZE on the query to view slice statistics for the query.

gp_motion_cost_per_row

Sets the Postgres Planner cost estimate for a Motion operator to transfer a row from one segment to another, measured as a fraction of the cost of a sequential page fetch. If 0, then the value used is two times the value of cpu_tuple_cost.

gp_print_create_gang_time

When a user starts a session with LightDB-A Database and issues a query, the system creates groups or 'gangs’ of worker processes on each segment to do the work. gp_print_create_gang_time controls the display of additional information about gang creation, including gang reuse status and the shortest and longest connection establishment time to the segment.

The default value is false, LightDB-A Database does not display the additional gang creation information.

gp_recursive_cte

Controls the availability of the RECURSIVE keyword in the WITH clause of a SELECT [INTO] command, or a DELETE, INSERT or UPDATE command. The keyword allows a subquery in the WITH clause of a command to reference itself. The default value is true, the RECURSIVE keyword is allowed in the WITH clause of a command.

For information about the RECURSIVE keyword, see the SELECT command and WITH Queries (Common Table Expressions).

The parameter can be set for a database system, an individual database, or a session or query.

Note This parameter was previously named gp_recursive_cte_prototype, but has been renamed to reflect the current status of the implementation.

gp_reject_percent_threshold

For single row error handling on COPY and external table SELECTs, sets the number of rows processed before SEGMENT REJECT LIMIT n PERCENT starts calculating.

gp_reraise_signal

If enabled, will attempt to dump core if a fatal server error occurs.

gp_resgroup_memory_policy

Note The gp_resgroup_memory_policy server configuration parameter is enforced only when resource group-based resource management is active.

Used by a resource group to manage memory allocation to query operators.

When set to auto, LightDB-A Database uses resource group memory limits to distribute memory across query operators, allocating a fixed size of memory to non-memory-intensive operators and the rest to memory-intensive operators.

When you specify eager_free, LightDB-A Database distributes memory among operators more optimally by re-allocating memory released by operators that have completed their processing to operators in a later query stage.

gp_resource_group_bypass

Note The gp_resource_group_bypass server configuration parameter is enforced only when resource group-based resource management is active.

Activates or deactivates the enforcement of resource group concurrent transaction limits on LightDB-A Database resources. The default value is false, which enforces resource group transaction limits. Resource groups manage resources such as CPU, memory, and the number of concurrent transactions that are used by queries and external components such as PL/Container.

You can set this parameter to true to bypass resource group concurrent transaction limitations so that a query can run immediately. For example, you can set the parameter to true for a session to run a system catalog query or a similar query that requires a minimal amount of resources.

When you set this parameter to true and a run a query, the query runs in this environment:

• The query runs inside a resource group. The resource group assignment for the query does not change.
• The query memory quota is approximately 10 MB per query. The memory is allocated from resource group shared memory or global shared memory. The query fails if there is not enough shared memory available to fulfill the memory allocation request.

This parameter can be set for a session. The parameter cannot be set within a transaction or a function.

gp_resource_group_cpu_ceiling_enforcement

Enables the Ceiling Enforcement mode when assigning CPU resources by Percentage. When deactivated, the Elastic mode will be used.

gp_resource_group_cpu_limit

Note The gp_resource_group_cpu_limit server configuration parameter is enforced only when resource group-based resource management is active.

Identifies the maximum percentage of system CPU resources to allocate to resource groups on each LightDB-A Database segment node.

gp_resource_group_enable_recalculate_query_mem

Note The gp_resource_group_enable_recalculate_query_mem server configuration parameter is enforced only when resource group-based resource management is active.

Specifies whether or not LightDB-A Database recalculates the maximum amount of memory to allocate per query running in a resource group. The default value is true, LightDB-A Database recalculates the maximum per-query memory based on the memory configuration and the number of primary segments on the segment host. When set to false, LightDB-A database calculates the maximum per-query memory based on the memory configuration and the number of primary segments on the coordinator host.

gp_resource_group_memory_limit

Note The gp_resource_group_memory_limit server configuration parameter is enforced only when resource group-based resource management is active.

Identifies the maximum percentage of system memory resources to allocate to resource groups on each LightDB-A Database segment node.

Note When resource group-based resource management is active, the memory allotted to a segment host is equally shared by active primary segments. LightDB-A Database assigns memory to primary segments when the segment takes the primary role. The initial memory allotment to a primary segment does not change, even in a failover situation. This may result in a segment host utilizing more memory than the gp_resource_group_memory_limit setting permits.

For example, suppose your LightDB-A Database cluster is utilizing the default gp_resource_group_memory_limit of 0.7 and a segment host named seghost1 has 4 primary segments and 4 mirror segments. LightDB-A Database assigns each primary segment on seghost1 (0.7 / 4 = 0.175) of overall system memory. If failover occurs and two mirrors on seghost1 fail over to become primary segments, each of the original 4 primaries retain their memory allotment of 0.175, and the two new primary segments are each allotted (0.7 / 6 = 0.116) of system memory. seghost1’s overall memory allocation in this scenario is

0.7 + (0.116 * 2) = 0.932

which is above the percentage configured in the gp_resource_group_memory_limit setting.

gp_resource_group_queuing_timeout

Note The gp_resource_group_queuing_timeout server configuration parameter is enforced only when resource group-based resource management is active.

Cancel a transaction queued in a resource group that waits longer than the specified number of milliseconds. The time limit applies separately to each transaction. The default value is zero; transactions are queued indefinitely and never time out.

gp_resource_manager

Identifies the resource management scheme currently enabled in the LightDB-A Database cluster. The default scheme is to use resource queues. For information about LightDB-A Database resource management, see Managing Resources.

gp_resqueue_memory_policy

Note The gp_resqueue_memory_policy server configuration parameter is enforced only when resource queue-based resource management is active.

Enables LightDB-A memory management features. The distribution algorithm eager_free takes advantage of the fact that not all operators run at the same time(in LightDB-A Database 4.2 and later). The query plan is divided into stages and LightDB-A Database eagerly frees memory allocated to a previous stage at the end of that stage’s execution, then allocates the eagerly freed memory to the new stage.

When set to none, memory management is the same as in LightDB-A Database releases prior to 4.1.

When set to auto, query memory usage is controlled by statement_mem and resource queue memory limits.

gp_resqueue_priority

Note The gp_resqueue_priority server configuration parameter is enforced only when resource queue-based resource management is active.

Activates or deactivates query prioritization. When this parameter is deactivated, existing priority settings are not evaluated at query run time.

gp_resqueue_priority_cpucores_per_segment

Note The gp_resqueue_priority_cpucores_per_segment server configuration parameter is enforced only when resource queue-based resource management is active.

Specifies the number of CPU units allocated to each segment instance on a segment host. If the segment is configured with primary-mirror segment instance pairs, use the number of primary segment instances on the host in the calculation. Include any CPU core that is available to the operating system, including virtual CPU cores, in the total number of available cores.

For example, if a LightDB-A Database cluster has 10-core segment hosts that are configured with four primary segments, set the value to 2.5 on each segment host (10 divided by 4). A coordinator host typically has only a single running coordinator instance, so set the value on the coordinator and standby maaster hosts to reflect the usage of all available CPU cores, in this case 10.

Incorrect settings can result in CPU under-utilization or query prioritization not working as designed.

gp_resqueue_priority_sweeper_interval

Note The gp_resqueue_priority_sweeper_interval server configuration parameter is enforced only when resource queue-based resource management is active.

Specifies the interval at which the sweeper process evaluates current CPU usage. When a new statement becomes active, its priority is evaluated and its CPU share determined when the next interval is reached.

gp_retrieve_conn

A session that you initiate with PGOPTIONS='-c gp_retrieve_conn=true' is a retrieve session. You use a retrieve session to retrieve query result tuples from a specific endpoint instantiated for a parallel retrieve cursor.

The default value is false.

gp_role

The role of this server process is set to dispatch for the coordinator and execute for a segment.

gp_segment_connect_timeout

Time that the LightDB-A interconnect will try to connect to a segment instance over the network before timing out. Controls the network connection timeout between coordinator and primary segments, and primary to mirror segment replication processes.

gp_segments_for_planner

Sets the number of primary segment instances for the Postgres Planner to assume in its cost and size estimates. If 0, then the value used is the actual number of primary segments. This variable affects the Postgres Planner’s estimates of the number of rows handled by each sending and receiving process in Motion operators.

gp_server_version

Reports the version number of the server as a string. A version modifier argument might be appended to the numeric portion of the version string, example: 5.0.0 beta.

gp_server_version_num

Reports the version number of the server as an integer. The number is guaranteed to always be increasing for each version and can be used for numeric comparisons. The major version is represented as is, the minor and patch versions are zero-padded to always be double digit wide.

gp_session_id

A system assigned ID number for a client session. Starts counting from 1 when the coordinator instance is first started.

gp_set_proc_affinity

If enabled, when a LightDB-A server process (postmaster) is started it will bind to a CPU.

gp_set_read_only

Set to on to deactivate writes to the database. Any in progress transactions must finish before read-only mode takes affect.

gp_statistics_pullup_from_child_partition

This parameter directs the Postgres Planner on where to obtain statistics when it plans a query on a partitioned table.

The default value is off, the Postgres Planner uses the statistics of the root partitioned table, if it has any, when it plans a query. If the root partitioned table has no statistics, the Planner attempts to use the statistics from the largest child partition.

When set to on, the Planner attempts to use the statistics from the largest child partition when it plans a query on a partitioned table.

gp_statistics_use_fkeys

When enabled, the Postgres Planner will use the statistics of the referenced column in the parent table when a column is foreign key reference to another table instead of the statistics of the column itself.

Note This parameter is deprecated and will be removed in a future LightDB-A Database release.

gp_use_legacy_hashops

For a table that is defined with a DISTRIBUTED BY key\_column clause, this parameter controls the hash algorithm that is used to distribute table data among segment instances. The default value is false, use the jump consistent hash algorithm.

Setting the value to true uses the modulo hash algorithm that is compatible with LightDB-A Database 5.x and earlier releases.

gp_vmem_idle_resource_timeout

If a database session is idle for longer than the time specified, the session will free system resources (such as shared memory), but remain connected to the database. This allows more concurrent connections to the database at one time.

gp_vmem_protect_limit

Note The gp_vmem_protect_limit server configuration parameter is enforced only when resource queue-based resource management is active.

Sets the amount of memory (in number of MBs) that all postgres processes of an active segment instance can consume. If a query causes this limit to be exceeded, memory will not be allocated and the query will fail. Note that this is a local parameter and must be set for every segment in the system (primary and mirrors). When setting the parameter value, specify only the numeric value. For example, to specify 4096MB, use the value 4096. Do not add the units MB to the value.

To prevent over-allocation of memory, these calculations can estimate a safe gp_vmem_protect_limit value.

First calculate the value of gp_vmem. This is the LightDB-A Database memory available on a host.

• If the total system memory is less than 256 GB, use this formula:

  gp_vmem = ((SWAP + RAM) – (7.5GB + 0.05 * RAM)) / 1.7
  

• If the total system memory is equal to or greater than 256 GB, use this formula:

  gp_vmem = ((SWAP + RAM) – (7.5GB + 0.05 * RAM)) / 1.17
  

where SWAP is the host swap space and RAM is the RAM on the host in GB.

Next, calculate the max_acting_primary_segments. This is the maximum number of primary segments that can be running on a host when mirror segments are activated due to a failure. With mirrors arranged in a 4-host block with 8 primary segments per host, for example, a single segment host failure would activate two or three mirror segments on each remaining host in the failed host’s block. The max_acting_primary_segments value for this configuration is 11 (8 primary segments plus 3 mirrors activated on failure).

This is the calculation for gp_vmem_protect_limit. The value should be converted to MB.

gp_vmem_protect_limit = <gp_vmem> / <acting_primary_segments>

For scenarios where a large number of workfiles are generated, this is the calculation for gp_vmem that accounts for the workfiles.

• If the total system memory is less than 256 GB:

  <gp_vmem> = ((<SWAP> + <RAM>) – (7.5GB + 0.05 * <RAM> - (300KB * <total_#_workfiles>))) / 1.7
  

• If the total system memory is equal to or greater than 256 GB:

  <gp_vmem> = ((<SWAP> + <RAM>) – (7.5GB + 0.05 * <RAM> - (300KB * <total_#_workfiles>))) / 1.17
  

For information about monitoring and managing workfile usage, see the LightDB-A Database Administrator Guide.

Based on the gp_vmem value you can calculate the value for the vm.overcommit_ratio operating system kernel parameter. This parameter is set when you configure each LightDB-A Database host.

vm.overcommit_ratio = (<RAM> - (0.026 * <gp_vmem>)) / <RAM>

Note The default value for the kernel parameter vm.overcommit_ratio in Red Hat Enterprise Linux is 50.

For information about the kernel parameter, see the LightDB-A Database Installation Guide.

gp_vmem_protect_segworker_cache_limit

If a query executor process consumes more than this configured amount, then the process will not be cached for use in subsequent queries after the process completes. Systems with lots of connections or idle processes may want to reduce this number to free more memory on the segments. Note that this is a local parameter and must be set for every segment.

gp_workfile_compression

Specifies whether the temporary files created, when a hash aggregation or hash join operation spills to disk, are compressed.

If your LightDB-A Database installation uses serial ATA (SATA) disk drives, enabling compression might help to avoid overloading the disk subsystem with IO operations.

gp_workfile_limit_files_per_query

Sets the maximum number of temporary spill files (also known as workfiles) allowed per query per segment. Spill files are created when running a query that requires more memory than it is allocated. The current query is terminated when the limit is exceeded.

Set the value to 0 (zero) to allow an unlimited number of spill files. coordinator session reload

gp_workfile_limit_per_query

Sets the maximum disk size an individual query is allowed to use for creating temporary spill files at each segment. The default value is 0, which means a limit is not enforced.

gp_workfile_limit_per_segment

Sets the maximum total disk size that all running queries are allowed to use for creating temporary spill files at each segment. The default value is 0, which means a limit is not enforced.

gpfdist_retry_timeout

Controls the time (in seconds) that LightDB-A Database waits before returning an error when LightDB-A Database is attempting to connect or write to a gpfdist server and gpfdist does not respond. The default value is 300 (5 minutes). A value of 0 deactivates the timeout.

ignore_checksum_failure

Only has effect if data_checksums is enabled.

LightDB-A Database uses checksums to prevent loading data that has been corrupted in the file system into memory managed by database processes.

By default, when a checksum verify error occurs when reading a heap data page, LightDB-A Database generates an error and prevents the page from being loaded into managed memory. When ignore_checksum_failure is set to on and a checksum verify failure occurs, LightDB-A Database generates a warning, and allows the page to be read into managed memory. If the page is then updated it is saved to disk and replicated to the mirror. If the page header is corrupt an error is reported even if this option is enabled.

Caution Setting ignore_checksum_failure to on may propagate or hide data corruption or lead to other serious problems. However, if a checksum failure has already been detected and the page header is uncorrupted, setting ignore_checksum_failure to on may allow you to bypass the error and recover undamaged tuples that may still be present in the table.

The default setting is off, and it can only be changed by a superuser.

integer_datetimes

Reports whether PostgreSQL was built with support for 64-bit-integer dates and times.

IntervalStyle

Sets the display format for interval values. The value sql_standard produces output matching SQL standard interval literals. The value postgres produces output matching PostgreSQL releases prior to 8.4 when the DateStyle parameter was set to ISO.

The value postgres_verbose produces output matching LightDB-A releases prior to 3.3 when the DateStyle parameter was set to non-ISO output.

The value iso_8601 will produce output matching the time interval format with designators defined in section 4.4.3.2 of ISO 8601. See the PostgreSQL 9.4 documentation for more information.

join_collapse_limit

The Postgres Planner will rewrite explicit inner JOIN constructs into lists of FROM items whenever a list of no more than this many items in total would result. By default, this variable is set the same as from_collapse_limit, which is appropriate for most uses. Setting it to 1 prevents any reordering of inner JOINs. Setting this variable to a value between 1 and from_collapse_limit might be useful to trade off planning time against the quality of the chosen plan (higher values produce better plans).

krb_caseins_users

Sets whether Kerberos user names should be treated case-insensitively. The default is case sensitive (off).

krb_server_keyfile

Sets the location of the Kerberos server key file.

lc_collate

Reports the locale in which sorting of textual data is done. The value is determined when the LightDB-A Database array is initialized.

lc_ctype

Reports the locale that determines character classifications. The value is determined when the LightDB-A Database array is initialized.

lc_messages

Sets the language in which messages are displayed. The locales available depends on what was installed with your operating system - use locale -a to list available locales. The default value is inherited from the execution environment of the server. On some systems, this locale category does not exist. Setting this variable will still work, but there will be no effect. Also, there is a chance that no translated messages for the desired language exist. In that case you will continue to see the English messages.

lc_monetary

Sets the locale to use for formatting monetary amounts, for example with the to_char family of functions. The locales available depends on what was installed with your operating system - use locale -a to list available locales. The default value is inherited from the execution environment of the server.

lc_numeric

Sets the locale to use for formatting numbers, for example with the to_char family of functions. The locales available depends on what was installed with your operating system - use locale -a to list available locales. The default value is inherited from the execution environment of the server.

lc_time

This parameter currently does nothing, but may in the future.

listen_addresses

Specifies the TCP/IP address(es) on which the server is to listen for connections from client applications - a comma-separated list of host names and/or numeric IP addresses. The special entry * corresponds to all available IP interfaces. If the list is empty, only UNIX-domain sockets can connect.

local_preload_libraries

Comma separated list of shared library files to preload at the start of a client session.

lock_timeout

Abort any statement that waits longer than the specified number of milliseconds while attempting to acquire a lock on a table, index, row, or other database object. The time limit applies separately to each lock acquisition attempt. The limit applies both to explicit locking requests (such as LOCK TABLE, or SELECT FOR UPDATE without NOWAIT) and to implicitly-acquired locks. If log_min_error_statement is set to ERROR or lower, LightDB-A Database logs the statement that timed out. A value of zero (the default) turns off this lock wait monitoring.

Unlike statement_timeout, this timeout can only occur while waiting for locks. Note that if statement_timeout is nonzero, it is rather pointless to set lock_timeout to the same or larger value, since the statement timeout would always trigger first.

LightDB-A Database uses the deadlock_timeout and gp_global_deadlock_detector_period to trigger local and global deadlock detection. Note that if lock_timeout is turned on and set to a value smaller than these deadlock detection timeouts, LightDB-A Database will abort a statement before it would ever trigger a deadlock check in that session.

Note Setting lock_timeout in postgresql.conf is not recommended because it would affect all sessions

log_autostats

Logs information about automatic ANALYZE operations related to gp_autostats_modeand gp_autostats_on_change_threshold.

log_connections

This outputs a line to the server log detailing each successful connection. Some client programs, like psql, attempt to connect twice while determining if a password is required, so duplicate “connection received” messages do not always indicate a problem.

log_disconnections

This outputs a line in the server log at termination of a client session, and includes the duration of the session.

log_dispatch_stats

When set to “on,” this parameter adds a log message with verbose information about the dispatch of the statement.

log_duration

Causes the duration of every completed statement which satisfies log_statement to be logged.

log_error_verbosity

Controls the amount of detail written in the server log for each message that is logged.

log_executor_stats

For each query, write performance statistics of the query executor to the server log. This is a crude profiling instrument. Cannot be enabled together with log_statement_stats.

log_file_mode

On Unix systems this parameter sets the permissions for log files when logging_collector is enabled. The parameter value is expected to be a numeric mode specified in the format accepted by the chmod and umask system calls.

log_hostname

By default, connection log messages only show the IP address of the connecting host. Turning on this option causes logging of the IP address and host name of the LightDB-A Database coordinator. Note that depending on your host name resolution setup this might impose a non-negligible performance penalty.

log_min_duration_statement

Logs the statement and its duration on a single log line if its duration is greater than or equal to the specified number of milliseconds. Setting this to 0 will print all statements and their durations. -1 deactivates the feature. For example, if you set it to 250 then all SQL statements that run 250ms or longer will be logged. Enabling this option can be useful in tracking down unoptimized queries in your applications.

log_min_error_statement

Controls whether or not the SQL statement that causes an error condition will also be recorded in the server log. All SQL statements that cause an error of the specified level or higher are logged. The default is ERROR. To effectively turn off logging of failing statements, set this parameter to PANIC.

log_min_messages

Controls which message levels are written to the server log. Each level includes all the levels that follow it. The later the level, the fewer messages are sent to the log.

If the LightDB-A Database PL/Container extension is installed. This parameter also controls the PL/Container log level. For information about the extension, see PL/pgSQL Language.

log_parser_stats

For each query, write performance statistics of the query parser to the server log. This is a crude profiling instrument. Cannot be enabled together with log_statement_stats.

log_planner_stats

For each query, write performance statistics of the Postgres Planner to the server log. This is a crude profiling instrument. Cannot be enabled together with log_statement_stats.

log_rotation_age

Determines the amount of time LightDB-A Database writes messages to the active log file. When this amount of time has elapsed, the file is closed and a new log file is created. Set to zero to deactivate time-based creation of new log files.

log_rotation_size

Determines the size of an individual log file that triggers rotation. When the log file size is equal to or greater than this size, the file is closed and a new log file is created. Set to zero to deactivate size-based creation of new log files.

The maximum value is INT_MAX/1024. If an invalid value is specified, the default value is used. INT_MAX is the largest value that can be stored as an integer on your system.

log_statement

Controls which SQL statements are logged. DDL logs all data definition commands like CREATE, ALTER, and DROP commands. MOD logs all DDL statements, plus INSERT, UPDATE, DELETE, TRUNCATE, and COPY FROM. PREPARE and EXPLAIN ANALYZE statements are also logged if their contained command is of an appropriate type.

log_statement_stats

For each query, write total performance statistics of the query parser, planner, and executor to the server log. This is a crude profiling instrument.

log_temp_files

Controls logging of temporary file names and sizes. Temporary files can be created for sorts, hashes, temporary query results and spill files. A log entry is made in log for each temporary file when it is deleted. Depending on the source of the temporary files, the log entry could be created on either the coordinator and/or segments. A log_temp_files value of zero logs all temporary file information, while positive values log only files whose size is greater than or equal to the specified number of kilobytes. The default setting is -1, which deactivates logging. Only superusers can change this setting.

log_timezone

Sets the time zone used for timestamps written in the log. Unlike TimeZone, this value is system-wide, so that all sessions will report timestamps consistently. The default is unknown, which means to use whatever the system environment specifies as the time zone.

log_truncate_on_rotation

Truncates (overwrites), rather than appends to, any existing log file of the same name. Truncation will occur only when a new file is being opened due to time-based rotation. For example, using this setting in combination with a log_filename such as gpseg#-%H.log would result in generating twenty-four hourly log files and then cyclically overwriting them. When off, pre-existing files will be appended to in all cases.

maintenance_work_mem

Specifies the maximum amount of memory to be used in maintenance operations, such as VACUUM and CREATE INDEX. It defaults to 16 megabytes (16MB). Larger settings might improve performance for vacuuming and for restoring database dumps.

max_connections

The maximum number of concurrent connections to the database server. In a LightDB-A Database system, user client connections go through the LightDB-A coordinator instance only. Segment instances should allow 3-10 times the amount as the coordinator. When you increase this parameter, max_prepared_transactions must be increased as well. For more information about limiting concurrent connections, see “Configuring Client Authentication” in the LightDB-A Database Administrator Guide.

Increasing this parameter may cause LightDB-A Database to request more shared memory. See shared_buffers for information about LightDB-A server instance shared memory buffers.

|local, system, restart|

max_files_per_process

Sets the maximum number of simultaneously open files allowed to each server subprocess. If the kernel is enforcing a safe per-process limit, you don’t need to worry about this setting. Some platforms such as BSD, the kernel will allow individual processes to open many more files than the system can really support.

max_function_args

Reports the maximum number of function arguments.

max_identifier_length

Reports the maximum identifier length.

max_index_keys

Reports the maximum number of index keys.

max_locks_per_transaction

The shared lock table is created with room to describe locks on max_locks_per_transaction * (max_connections + max_prepared_transactions) objects, so no more than this many distinct objects can be locked at any one time. This is not a hard limit on the number of locks taken by any one transaction, but rather a maximum average value. You might need to raise this value if you have clients that touch many different tables in a single transaction.

max_prepared_transactions

Sets the maximum number of transactions that can be in the prepared state simultaneously. LightDB-A uses prepared transactions internally to ensure data integrity across the segments. This value must be at least as large as the value of max_connections on the coordinator. Segment instances should be set to the same value as the coordinator.

max_resource_portals_per_transaction

Note The max_resource_portals_per_transaction server configuration parameter is enforced only when resource queue-based resource management is active.

Sets the maximum number of simultaneously open user-declared cursors allowed per transaction. Note that an open cursor will hold an active query slot in a resource queue. Used for resource management.

max_resource_queues

Note The max_resource_queues server configuration parameter is enforced only when resource queue-based resource management is active.

Sets the maximum number of resource queues that can be created in a LightDB-A Database system. Note that resource queues are system-wide (as are roles) so they apply to all databases in the system.

max_stack_depth

Specifies the maximum safe depth of the server’s execution stack. The ideal setting for this parameter is the actual stack size limit enforced by the kernel (as set by ulimit -s or local equivalent), less a safety margin of a megabyte or so. Setting the parameter higher than the actual kernel limit will mean that a runaway recursive function can crash an individual backend process.

max_statement_mem

Sets the maximum memory limit for a query. Helps avoid out-of-memory errors on a segment host during query processing as a result of setting statement_mem too high.

Taking into account the configuration of a single segment host, calculate max_statement_mem as follows:

(seghost_physical_memory) / (average_number_concurrent_queries)

When changing both max_statement_mem and statement_mem, max_statement_mem must be changed first, or listed first in the postgresql.conf file.

memory_spill_ratio

Note The memory_spill_ratio server configuration parameter is enforced only when resource group-based resource management is active.

Sets the memory usage threshold percentage for memory-intensive operators in a transaction. When a transaction reaches this threshold, it spills to disk.

The default memory_spill_ratio percentage is the value defined for the resource group assigned to the currently active role. You can set memory_spill_ratio at the session level to selectively set this limit on a per-query basis. For example, if you have a specific query that spills to disk and requires more memory, you may choose to set a larger memory_spill_ratio to increase the initial memory allocation.

You can specify an integer percentage value from 0 to 100 inclusive. If you specify a value of 0, LightDB-A Database uses the statement_mem server configuration parameter value to control the initial query operator memory amount.

optimizer

Activates or deactivates GPORCA when running SQL queries. The default is on. If you deactivate GPORCA, LightDB-A Database uses only the Postgres Planner.

GPORCA co-exists with the Postgres Planner. With GPORCA enabled, LightDB-A Database uses GPORCA to generate an execution plan for a query when possible. If GPORCA cannot be used, then the Postgres Planner is used.

The optimizer parameter can be set for a database system, an individual database, or a session or query.

For information about the Postgres Planner and GPORCA, see Querying Data in the LightDB-A Database Administrator Guide.

optimizer_analyze_root_partition

For a partitioned table, controls whether the ROOTPARTITION keyword is required to collect root partition statistics when the ANALYZE command is run on the table. GPORCA uses the root partition statistics when generating a query plan. The Postgres Planner does not use these statistics.

The default setting for the parameter is on, the ANALYZE command can collect root partition statistics without the ROOTPARTITION keyword. Root partition statistics are collected when you run ANALYZE on the root partition, or when you run ANALYZE on a child leaf partition of the partitioned table and the other child leaf partitions have statistics. When the value is off, you must run ANALZYE ROOTPARTITION to collect root partition statistics.

When the value of the server configuration parameter optimizer is on (the default), the value of this parameter should also be on. For information about collecting table statistics on partitioned tables, see ANALYZE.

For information about the Postgres Planner and GPORCA, see Querying Data in the LightDB-A Database Administrator Guide.

optimizer_array_expansion_threshold

When GPORCA is enabled (the default) and is processing a query that contains a predicate with a constant array, the optimizer_array_expansion_threshold parameter limits the optimization process based on the number of constants in the array. If the array in the query predicate contains more than the number elements specified by parameter, GPORCA deactivates the transformation of the predicate into its disjunctive normal form during query optimization.

The default value is 20.

For example, when GPORCA is running a query that contains an IN clause with more than 20 elements, GPORCA does not transform the predicate into its disjunctive normal form during query optimization to reduce optimization time consume less memory. The difference in query processing can be seen in the filter condition for the IN clause of the query EXPLAIN plan.

Changing the value of this parameter changes the trade-off between a shorter optimization time and lower memory consumption, and the potential benefits from constraint derivation during query optimization, for example conflict detection and partition elimination.

The parameter can be set for a database system, an individual database, or a session or query.

optimizer_control

Controls whether the server configuration parameter optimizer can be changed with SET, the RESET command, or the LightDB-A Database utility gpconfig. If the optimizer_control parameter value is on, users can set the optimizer parameter. If the optimizer_control parameter value is off, the optimizer parameter cannot be changed.

optimizer_cost_model

When GPORCA is enabled (the default), this parameter controls the cost model that GPORCA chooses for bitmap scans used with bitmap indexes or with btree indexes on AO tables.

• legacy - preserves the calibrated cost model used by GPORCA in LightDB-A Database releases 6.13 and earlier
• calibrated - improves cost estimates for indexes
• experimental - reserved for future experimental cost models; currently equivalent to the calibrated model

The default cost model, calibrated, is more likely to choose a faster bitmap index with nested loop joins instead of hash joins.

optimizer_cte_inlining_bound

When GPORCA is enabled (the default), this parameter controls the amount of inlining performed for common table expression (CTE) queries (queries that contain a WHERE clause). The default value, 0, deactivates inlining.

The parameter can be set for a database system, an individual database, or a session or query.

optimizer_dpe_stats

When GPORCA is enabled (the default) and this parameter is true (the default), GPORCA derives statistics that allow it to more accurately estimate the number of rows to be scanned during dynamic partition elimination.

The parameter can be set for a database system, an individual database, or a session or query.

optimizer_discard_redistribute_hashjoin

When GPORCA is enabled (the default), this parameter specifies whether the Query Optimizer should eliminate plans that include a HashJoin operator with a Redistribute Motion child. Eliminating such plans can improve performance in cases where the data being joined exhibits high skewness in the join keys.

The default setting is off, GPORCA considers all plan alternatives, including those with a Redistribute Motion child, in the HashJoin operator. If you observe performance issues with queries that use a HashJoin with highly skewed data, you may want to consider setting optimizer_discard_redistribute_hashjoin to on to instruct GPORCA to discard such plans.

optimizer_enable_associativity

When GPORCA is enabled (the default), this parameter controls whether the join associativity transform is enabled during query optimization. The transform analyzes join orders. For the default value off, only the GPORCA dynamic programming algorithm for analyzing join orders is enabled. The join associativity transform largely duplicates the functionality of the newer dynamic programming algorithm.

If the value is on, GPORCA can use the associativity transform during query optimization.

The parameter can be set for a database system, an individual database, or a session or query.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_enable_dml

When GPORCA is enabled (the default) and this parameter is true (the default), GPORCA attempts to run DML commands such as INSERT, UPDATE, and DELETE. If GPORCA cannot run the command, LightDB-A Database falls back to the Postgres Planner.

When set to false, LightDB-A Database always falls back to the Postgres Planner when performing DML commands.

The parameter can be set for a database system, an individual database, or a session or query.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_enable_indexonlyscan

When GPORCA is enabled (the default) and this parameter is true (the default), GPORCA can generate index-only scan plan types for B-tree indexes. GPORCA accesses the index values only, not the data blocks of the relation. This provides a query execution performance improvement, particularly when the table has been vacuumed, has wide columns, and GPORCA does not need to fetch any data blocks (for example, they are visible).

When deactivated (false), GPORCA does not generate index-only scan plan types.

The parameter can be set for a database system, an individual database, or a session or query.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_enable_master_only_queries

When GPORCA is enabled (the default), this parameter allows GPORCA to run catalog queries that run only on the LightDB-A Database coordinator. For the default value off, only the Postgres Planner can run catalog queries that run only on the LightDB-A Database coordinator.

The parameter can be set for a database system, an individual database, or a session or query.

Note Enabling this parameter decreases performance of short running catalog queries. To avoid this issue, set this parameter only for a session or a query.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_enable_multiple_distinct_aggs

When GPORCA is enabled (the default), this parameter allows GPORCA to support Multiple Distinct Qualified Aggregates, such as SELECT count(DISTINCT a),sum(DISTINCT b) FROM foo. This parameter is deactivated by default because its plan is generally suboptimal in comparison to the plan generated by the Postgres planner.

The parameter can be set for a database system, an individual database, or a session or query.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_enable_replicated_table

When GPORCA is enabled (the default), this parameter controls GPORCA’s behavior when it encounters DML operations on a replicated table.

The default value is on, GPORCA attempts to plan and execute operations on replicated tables. When off, GPORCA immediately falls back to the Postgres Planner when it detects replicated table operations.

The parameter can be set for a database system, an individual database, or a session or query.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_force_agg_skew_avoidance

When GPORCA is enabled (the default), this parameter affects the query plan alternatives that GPORCA considers when 3 stage aggregate plans are generated. When the value is true, the default, GPORCA considers only 3 stage aggregate plans where the intermediate aggregation uses the GROUP BY and DISTINCT columns for distribution to reduce the effects of processing skew.

If the value is false, GPORCA can also consider a plan that uses GROUP BY columns for distribution. These plans might perform poorly when processing skew is present.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_force_comprehensive_join_implementation

When GPORCA is enabled (the default), this parameter affects its consideration of nested loop join and hash join alternatives.

The default value is false, GPORCA does not consider nested loop join alternatives when a hash join is available, which significantly improves optimization performance for most queries. When set to true, GPORCA will explore nested loop join alternatives even when a hash join is possible.

optimizer_force_multistage_agg

For the default settings, GPORCA is enabled and this parameter is false, GPORCA makes a cost-based choice between a one- or two-stage aggregate plan for a scalar distinct qualified aggregate. When true, GPORCA chooses a multi-stage aggregate plan when such a plan alternative is generated.

The parameter can be set for a database system, an individual database, or a session or query.

optimizer_force_three_stage_scalar_dqa

For the default settings, GPORCA is enabled and this parameter is true, GPORCA chooses a plan with multistage aggregates when such a plan alternative is generated. When the value is false, GPORCA makes a cost based choice rather than a heuristic choice.

The parameter can be set for a database system, an individual database, or a session, or query.

optimizer_join_arity_for_associativity_commutativity

The value is an optimization hint to limit the number of join associativity and join commutativity transformations explored during query optimization. The limit controls the alternative plans that GPORCA considers during query optimization. For example, the default value of 18 is an optimization hint for GPORCA to stop exploring join associativity and join commutativity transformations when an n-ary join operator has more than 18 children during optimization.

For a query with a large number of joins, specifying a lower value improves query performance by limiting the number of alternate query plans that GPORCA evaluates. However, setting the value too low might cause GPORCA to generate a query plan that performs sub-optimally.

This parameter has no effect when the optimizer_join_order parameter is set to query or greedy.

This parameter can be set for a database system or a session.

optimizer_join_order

When GPORCA is enabled, this parameter sets the optimization level for join ordering during query optimization by specifying which types of join ordering alternatives to evaluate.

• query - Uses the join order specified in the query.
• greedy - Evaluates the join order specified in the query and alternatives based on minimum cardinalities of the relations in the joins.
• exhaustive - Applies transformation rules to find and evaluate all join ordering alternatives.

The default value is exhaustive. Setting this parameter to query or greedy can generate a suboptimal query plan. However, if the administrator is confident that a satisfactory plan is generated with the query or greedy setting, query optimization time can be improved by setting the parameter to the lower optimization level.

Setting this parameter to query or greedy overrides the optimizer_join_order_threshold and optimizer_join_arity_for_associativity_commutativity parameters.

This parameter can be set for an individual database, a session, or a query.

optimizer_join_order_threshold

When GPORCA is enabled (the default), this parameter sets the maximum number of join children for which GPORCA will use the dynamic programming-based join ordering algorithm. You can set this value for a single query or for an entire session.

This parameter has no effect when the optimizer_join_query parameter is set to query or greedy.

optimizer_mdcache_size

Sets the maximum amount of memory on the LightDB-A Database coordinator that GPORCA uses to cache query metadata (optimization data) during query optimization. The memory limit session based. GPORCA caches query metadata during query optimization with the default settings: GPORCA is enabled and optimizer_metadata_caching is on.

The default value is 16384 (16MB). This is an optimal value that has been determined through performance analysis.

You can specify a value in KB, MB, or GB. The default unit is KB. For example, a value of 16384 is 16384KB. A value of 1GB is the same as 1024MB or 1048576KB. If the value is 0, the size of the cache is not limited.

This parameter can be set for a database system, an individual database, or a session or query.

optimizer_metadata_caching

When GPORCA is enabled (the default), this parameter specifies whether GPORCA caches query metadata (optimization data) in memory on the LightDB-A Database coordinator during query optimization. The default for this parameter is on, enable caching. The cache is session based. When a session ends, the cache is released. If the amount of query metadata exceeds the cache size, then old, unused metadata is evicted from the cache.

If the value is off, GPORCA does not cache metadata during query optimization.

This parameter can be set for a database system, an individual database, or a session or query.

The server configuration parameter optimizer_mdcache_size controls the size of the query metadata cache.

optimizer_minidump

GPORCA generates minidump files to describe the optimization context for a given query. The information in the file is not in a format that can be easily used for debugging or troubleshooting. The minidump file is located under the coordinator data directory and uses the following naming format:

Minidump_date_time.mdp

The minidump file contains this query related information:

• Catalog objects including data types, tables, operators, and statistics required by GPORCA
• An internal representation (DXL) of the query
• An internal representation (DXL) of the plan produced by GPORCA
• System configuration information passed to GPORCA such as server configuration parameters, cost and statistics configuration, and number of segments
• A stack trace of errors generated while optimizing the query

Setting this parameter to ALWAYS generates a minidump for all queries. Set this parameter to ONERROR to minimize total optimization time.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_nestloop_factor

This parameter adds a costing factor to GPORCA to prioritize hash joins instead of nested loop joins during query optimization. The default value of 1024 was chosen after evaluating numerous workloads with uniformly distributed data. 1024 should be treated as the practical upper bound setting for this parameter. If you find the GPORCA selects hash joins more often than it should, reduce the value to shift the costing factor in favor of nested loop joins.

The parameter can be set for a database system, an individual database, or a session or query.

optimizer_parallel_union

When GPORCA is enabled (the default), optimizer_parallel_union controls the amount of parallelization that occurs for queries that contain a UNION or UNION ALL clause.

When the value is off, the default, GPORCA generates a query plan where each child of an APPEND(UNION) operator is in the same slice as the APPEND operator. During query execution, the children are run in a sequential manner.

When the value is on, GPORCA generates a query plan where a redistribution motion node is under an APPEND(UNION) operator. During query execution, the children and the parent APPEND operator are on different slices, allowing the children of the APPEND(UNION) operator to run in parallel on segment instances.

The parameter can be set for a database system, an individual database, or a session or query.

optimizer_penalize_skew

When GPORCA is enabled (the default), this parameter allows GPORCA to penalize the local cost of a HashJoin with a skewed Redistribute Motion as child to favor a Broadcast Motion during query optimization. The default value is true.

GPORCA determines there is skew for a Redistribute Motion when the NDV (number of distinct values) is less than the number of segments.

The parameter can be set for a database system, an individual database, or a session or query.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_print_missing_stats

When GPORCA is enabled (the default), this parameter controls the display of table column information about columns with missing statistics for a query. The default value is true, display the column information to the client. When the value is false, the information is not sent to the client.

The information is displayed during query execution, or with the EXPLAIN or EXPLAIN ANALYZE commands.

The parameter can be set for a database system, an individual database, or a session.

optimizer_print_optimization_stats

When GPORCA is enabled (the default), this parameter enables logging of GPORCA query optimization statistics for various optimization stages for a query. The default value is off, do not log optimization statistics. To log the optimization statistics, this parameter must be set to on and the parameter client_min_messages must be set to log.

• set optimizer_print_optimization_stats = on;
• set client_min_messages = 'log';

The information is logged during query execution, or with the EXPLAIN or EXPLAIN ANALYZE commands.

This parameter can be set for a database system, an individual database, or a session or query.

optimizer_skew_factor

When GPORCA is enabled (the default), optimizer_skew_factor controls skew ratio computation.

The default value is 0, skew computation is turned off for GPORCA. To enable skew computation, set optimizer_skew_factor to a value between 1 and 100, inclusive.

The larger the optimizer_skew_factor, the larger the cost that GPORCA assigns to redistributed hash join, such that GPORCA favors more a broadcast hash join.

The parameter can be set for a database system, an individual database, or a session or query.

optimizer_sort_factor

When GPORCA is enabled (the default), optimizer_sort_factor controls the cost factor to apply to sorting operations during query optimization. The default value 1 specifies the default sort cost factor. The value is a ratio of increase or decrease from the default factor. For example, a value of 2.0 sets the cost factor at twice the default, and a value of 0.5 sets the factor at half the default.

The parameter can be set for a database system, an individual database, or a session or query.

optimizer_use_gpdb_allocators

When GPORCA is enabled (the default) and this parameter is true (the default), GPORCA uses LightDB-A Database memory management when running queries. When set to false, GPORCA uses GPORCA-specific memory management. LightDB-A Database memory management allows for faster optimization, reduced memory usage during optimization, and improves GPORCA support of vmem limits when compared to GPORCA-specific memory management.

For information about GPORCA, see About GPORCA in the LightDB-A Database Administrator Guide.

optimizer_xform_bind_threshold

When GPORCA is enabled (the default), this parameter controls the maximum number of bindings per transform that GPORCA produces per group expression. Setting this parameter limits the number of alternatives that GPORCA creates, in many cases reducing the optimization time and overall memory usage of queries that include deeply nested expressions.

The default value is 0, GPORCA produces an unlimited set of bindings.

password_encryption

When a password is specified in CREATE USER or ALTER USER without writing either ENCRYPTED or UNENCRYPTED, this option determines whether the password is to be encrypted.

plan_cache_mode

Prepared statements (either explicitly prepared or implicitly generated, for example by PL/pgSQL) can be run using custom or generic plans. Custom plans are created for each execution using its specific set of parameter values, while generic plans do not rely on the parameter values and can be re-used across executions. The use of a generic plan saves planning time, but if the ideal plan depends strongly on the parameter values, then a generic plan might be inefficient. The choice between these options is normally made automatically, but it can be overridden by setting the plan_cache_mode parameter. If the prepared statement has no parameters, a generic plan is always used.

The allowed values are auto (the default), force_custom_plan and force_generic_plan. This setting is considered when a cached plan is to be run, not when it is prepared. For more information see PREPARE.

The parameter can be set for a database system, an individual database, a session, or a query.

pljava_classpath

A colon (:) separated list of jar files or directories containing jar files needed for PL/Java functions. The full path to the jar file or directory must be specified, except the path can be omitted for jar files in the $GPHOME/lib/postgresql/java directory. The jar files must be installed in the same locations on all LightDB-A hosts and readable by the gpadmin user.

The pljava_classpath parameter is used to assemble the PL/Java classpath at the beginning of each user session. Jar files added after a session has started are not available to that session.

If the full path to a jar file is specified in pljava_classpath it is added to the PL/Java classpath. When a directory is specified, any jar files the directory contains are added to the PL/Java classpath. The search does not descend into subdirectories of the specified directories. If the name of a jar file is included in pljava_classpath with no path, the jar file must be in the $GPHOME/lib/postgresql/java directory.

Note Performance can be affected if there are many directories to search or a large number of jar files.

If pljava_classpath_insecure is false, setting the pljava_classpath parameter requires superuser privilege. Setting the classpath in SQL code will fail when the code is run by a user without superuser privilege. The pljava_classpath parameter must have been set previously by a superuser or in the postgresql.conf file. Changing the classpath in the postgresql.conf file requires a reload (gpstop -u).

pljava_classpath_insecure

Controls whether the server configuration parameter pljava_classpath can be set by a user without LightDB-A Database superuser privileges. When true, pljava_classpath can be set by a regular user. Otherwise, pljava_classpath can be set only by a database superuser. The default is false.

Caution Enabling this parameter exposes a security risk by giving non-administrator database users the ability to run unauthorized Java methods.

pljava_statement_cache_size

Sets the size in KB of the JRE MRU (Most Recently Used) cache for prepared statements.

pljava_release_lingering_savepoints

If true, lingering savepoints used in PL/Java functions will be released on function exit. If false, savepoints will be rolled back.

pljava_vmoptions

Defines the startup options for the Java VM. The default value is an empty string (“”).

port

The database listener port for a LightDB-A instance. The coordinator and each segment has its own port. Port numbers for the LightDB-A system must also be changed in the gp_segment_configuration catalog. You must shut down your LightDB-A Database system before changing port numbers.

quote_all_identifiers

Ensures that all identifiers are quoted, even if they are not keywords, when the database generates SQL. See also the --quote-all-identifiers option of pg_dump and pg_dumpall.

random_page_cost

Sets the estimate of the cost of a nonsequentially fetched disk page for the Postgres Planner. This is measured as a multiple of the cost of a sequential page fetch. A higher value makes it more likely a sequential scan will be used, a lower value makes it more likely an index scan will be used.

readable_external_table_timeout

When an SQL query reads from an external table, the parameter value specifies the amount of time in seconds that LightDB-A Database waits before cancelling the query when data stops being returned from the external table.

The default value of 0, specifies no time out. LightDB-A Database does not cancel the query.

If queries that use gpfdist run a long time and then return the error “intermittent network connectivity issues”, you can specify a value for readable_external_table_timeout. If no data is returned by gpfdist for the specified length of time, LightDB-A Database cancels the query.

repl_catchup_within_range

For LightDB-A Database coordinator mirroring, controls updates to the active coordinator. If the number of WAL segment files that have not been processed by the walsender exceeds this value, LightDB-A Database updates the active coordinator.

If the number of segment files does not exceed the value, LightDB-A Database blocks updates to the to allow the walsender process the files. If all WAL segments have been processed, the active coordinator is updated.

wal_sender_timeout

For LightDB-A Database coordinator mirroring, sets the maximum time in milliseconds that the walsender process on the active coordinator waits for a status message from the walreceiver process on the standby coordinator. If a message is not received, the walsender logs an error message.

The wal_receiver_status_interval controls the interval between walreceiver status messages.

resource_cleanup_gangs_on_wait

Note The resource_cleanup_gangs_on_wait server configuration parameter is enforced only when resource queue-based resource management is active.

If a statement is submitted through a resource queue, clean up any idle query executor worker processes before taking a lock on the resource queue.

resource_select_only

Note The resource_select_only server configuration parameter is enforced only when resource queue-based resource management is active.

Sets the types of queries managed by resource queues. If set to on, then SELECT, SELECT INTO, CREATE TABLE AS SELECT, and DECLARE CURSOR commands are evaluated. If set to off INSERT, UPDATE, and DELETE commands will be evaluated as well.

row_security

Note This configuration parameter has no effect on roles which bypass every row security policy, for example, superusers and roles configured with the BYPASSRLS attribute.

Controls whether to raise an error in lieu of applying a row security policy. When set to on, policies apply normally. When set to off, queries which would otherwise apply at least one policy fail.

The default is on. Change row_security to off where limited row visibility could cause incorrect results; for example, pg_dump makes that change by default.

For more information about row-level security policies, see CREATE POLICY.

runaway_detector_activation_percent

For queries that are managed by resource queues or resource groups, this parameter determines when LightDB-A Database terminates running queries based on the amount of memory the queries are using. A value of 100 deactivates the automatic termination of queries based on the percentage of memory that is utilized.

Either the resource queue or the resource group management scheme can be active in LightDB-A Database; both schemes cannot be active at the same time. The server configuration parameter gp_resource_manager controls which scheme is active.

When resource queues are enabled - This parameter sets the percent of utilized LightDB-A Database vmem memory that triggers the termination of queries. If the percentage of vmem memory that is utilized for a LightDB-A Database segment exceeds the specified value, LightDB-A Database terminates queries managed by resource queues based on memory usage, starting with the query consuming the largest amount of memory. Queries are terminated until the percentage of utilized vmem is below the specified percentage.

Specify the maximum vmem value for active LightDB-A Database segment instances with the server configuration parameter gp_vmem_protect_limit.

For example, if vmem memory is set to 10GB, and this parameter is 90 (90%), LightDB-A Database starts terminating queries when the utilized vmem memory exceeds 9 GB.

For information about resource queues, see Using Resource Queues.

When resource groups are enabled - This parameter sets the percent of utilized resource group global shared memory that triggers the termination of queries that are managed by resource groups that are configured to use the vmtracker memory auditor, such as admin_group and default_group. For information about memory auditors, see Memory Auditor.

Resource groups have a global shared memory pool when the sum of the MEMORY_LIMIT attribute values configured for all resource groups is less than 100. For example, if you have 3 resource groups configured with memory_limit values of 10 , 20, and 30, then global shared memory is 40% = 100% - (10% + 20% + 30%). See Global Shared Memory.

If the percentage of utilized global shared memory exceeds the specified value, LightDB-A Database terminates queries based on memory usage, selecting from queries managed by the resource groups that are configured to use the vmtracker memory auditor. LightDB-A Database starts with the query consuming the largest amount of memory. Queries are terminated until the percentage of utilized global shared memory is below the specified percentage.

For example, if global shared memory is 10GB, and this parameter is 90 (90%), LightDB-A Database starts terminating queries when the utilized global shared memory exceeds 9 GB.

For information about resource groups, see Using Resource Groups.

search_path

Specifies the order in which schemas are searched when an object is referenced by a simple name with no schema component. When there are objects of identical names in different schemas, the one found first in the search path is used. The system catalog schema, pg_catalog, is always searched, whether it is mentioned in the path or not. When objects are created without specifying a particular target schema, they will be placed in the first schema listed in the search path. The current effective value of the search path can be examined via the SQL function current_schema(). current_schema() shows how the requests appearing in search_path were resolved.

seq_page_cost

For the Postgres Planner, sets the estimate of the cost of a disk page fetch that is part of a series of sequential fetches.

server_encoding

Reports the database encoding (character set). It is determined when the LightDB-A Database array is initialized. Ordinarily, clients need only be concerned with the value of client_encoding.

server_version

Reports the version of PostgreSQL that this release of LightDB-A Database is based on.

server_version_num

Reports the version of PostgreSQL that this release of LightDB-A Database is based on as an integer.

shared_buffers

Sets the amount of memory a LightDB-A Database segment instance uses for shared memory buffers. This setting must be at least 128KB and at least 16KB times max_connections.

Each LightDB-A Database segment instance calculates and attempts to allocate certain amount of shared memory based on the segment configuration. The value of shared_buffers is significant portion of this shared memory calculation, but is not all it. When setting shared_buffers, the values for the operating system parameters SHMMAX or SHMALL might also need to be adjusted.

The operating system parameter SHMMAX specifies maximum size of a single shared memory allocation. The value of SHMMAX must be greater than this value:

 `shared_buffers` + <other_seg_shmem>

The value of other_seg_shmem is the portion the LightDB-A Database shared memory calculation that is not accounted for by the shared_buffers value. The other_seg_shmem value will vary based on the segment configuration.

With the default LightDB-A Database parameter values, the value for other_seg_shmem is approximately 111MB for LightDB-A Database segments and approximately 79MB for the LightDB-A Database coordinator.

The operating system parameter SHMALL specifies the maximum amount of shared memory on the host. The value of SHMALL must be greater than this value:

 (<num_instances_per_host> * ( `shared_buffers` + <other_seg_shmem> )) + <other_app_shared_mem> 

The value of other_app_shared_mem is the amount of shared memory that is used by other applications and processes on the host.

When shared memory allocation errors occur, possible ways to resolve shared memory allocation issues are to increase SHMMAX or SHMALL, or decrease shared_buffers or max_connections.

See the LightDB-A Database Installation Guide for information about the LightDB-A Database values for the parameters SHMMAX and SHMALL.

shared_preload_libraries

A comma-separated list of shared libraries that are to be preloaded at server start. PostgreSQL procedural language libraries can be preloaded in this way, typically by using the syntax ’$libdir/plXXX’ where XXX is pgsql, perl, tcl, or python. By preloading a shared library, the library startup time is avoided when the library is first used. If a specified library is not found, the server will fail to start.

Note When you add a library to shared_preload_libraries, be sure to retain any previous setting of the parameter.

ssl

Enables SSL connections.

ssl_ciphers

Specifies a list of SSL ciphers that are allowed to be used on secure connections. ssl_ciphers overrides any ciphers string specified in /etc/openssl.cnf. The default value ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH enables all ciphers except for ADH, LOW, EXP, and MD5 ciphers, and prioritizes ciphers by their strength.

Note With TLS 1.2 some ciphers in MEDIUM and HIGH strength still use NULL encryption (no encryption for transport), which the default ssl_ciphers string allows. To bypass NULL ciphers with TLS 1.2 use a string such as TLSv1.2:!eNULL:!aNULL.

See the openssl manual page for a list of supported ciphers.

standard_conforming_strings

Determines whether ordinary string literals (’…’) treat backslashes literally, as specified in the SQL standard. The default value is on. Turn this parameter off to treat backslashes in string literals as escape characters instead of literal backslashes. Applications may check this parameter to determine how string literals are processed. The presence of this parameter can also be taken as an indication that the escape string syntax (E’…’) is supported.

statement_mem

Allocates segment host memory per query. The amount of memory allocated with this parameter cannot exceed max_statement_mem or the memory limit on the resource queue or resource group through which the query was submitted. If additional memory is required for a query, temporary spill files on disk are used.

If you are using resource groups to control resource allocation in your LightDB-A Database cluster:

• LightDB-A Database uses statement_mem to control query memory usage when the resource group MEMORY_SPILL_RATIO is set to 0.

• You can use the following calculation to estimate a reasonable statement_mem value:

  rg_perseg_mem = ((RAM * (vm.overcommit_ratio / 100) + SWAP) * gp_resource_group_memory_limit) / num_active_primary_segments
  statement_mem = rg_perseg_mem / max_expected_concurrent_queries
  

If you are using resource queues to control resource allocation in your LightDB-A Database cluster:

• When gp_resqueue_memory_policy =auto, statement_mem and resource queue memory limits control query memory usage.

• You can use the following calculation to estimate a reasonable statement_mem value for a wide variety of situations:

  ( <gp_vmem_protect_limit>GB * .9 ) / <max_expected_concurrent_queries>
  

  For example, with a gp_vmem_protect_limit set to 8192MB (8GB) and assuming a maximum of 40 concurrent queries with a 10% buffer, you would use the following calculation to determine the statement_mem value:

  (8GB * .9) / 40 = .18GB = 184MB
  

When changing both max_statement_mem and statement_mem, max_statement_mem must be changed first, or listed first in the postgresql.conf file.

statement_timeout

Abort any statement that takes over the specified number of milliseconds. 0 turns off the limitation.

stats_queue_level

Note The stats_queue_level server configuration parameter is enforced only when resource queue-based resource management is active.

Collects resource queue statistics on database activity.

superuser_reserved_connections

Determines the number of connection slots that are reserved for LightDB-A Database superusers.

tcp_keepalives_count

How many keepalives may be lost before the connection is considered dead. A value of 0 uses the system default. If TCP_KEEPCNT is not supported, this parameter must be 0.

Use this parameter for all connections that are not between a primary and mirror segment.

tcp_keepalives_idle

Number of seconds between sending keepalives on an otherwise idle connection. A value of 0 uses the system default. If TCP_KEEPIDLE is not supported, this parameter must be 0.

Use this parameter for all connections that are not between a primary and mirror segment.

tcp_keepalives_interval

How many seconds to wait for a response to a keepalive before retransmitting. A value of 0 uses the system default. If TCP_KEEPINTVL is not supported, this parameter must be 0.

Use this parameter for all connections that are not between a primary and mirror segment.

temp_buffers

Sets the maximum memory, in blocks, to allow for temporary buffers by each database session. These are session-local buffers used only for access to temporary tables. The setting can be changed within individual sessions, but only up until the first use of temporary tables within a session. The cost of setting a large value in sessions that do not actually need a lot of temporary buffers is only a buffer descriptor for each block, or about 64 bytes, per increment. However if a buffer is actually used, an additional 32768 bytes will be consumed.

You can set this parameter to the number of 32K blocks (for example, 1024 to allow 32MB for buffers), or specify the maximum amount of memory to allow (for example '48MB' for 1536 blocks). The gpconfig utility and SHOW command report the maximum amount of memory allowed for temporary buffers.

temp_tablespaces

Specifies tablespaces in which to create temporary objects (temp tables and indexes on temp tables) when a CREATE command does not explicitly specify a tablespace. These tablespaces can also include temporary files for purposes such as large data set sorting.

The value is a comma-separated list of tablespace names. When the list contains more than one tablespace name, LightDB-A chooses a random list member each time it creates a temporary object. An exception applies within a transaction, where successively created temporary objects are placed in successive tablespaces from the list. If the selected element of the list is an empty string, LightDB-A automatically uses the default tablespace of the current database instead.

When setting temp_tablespaces interactively, avoid specifying a nonexistent tablespace, or a tablespace for which the user does have CREATE privileges. For non-superusers, a superuser must GRANT them the CREATE privilege on the temp tablespace.. When using a previously set value (for example a value in postgresql.conf), nonexistent tablespaces are ignored, as are tablespaces for which the user lacks CREATE privilege.

The default value is an empty string, which results in all temporary objects being created in the default tablespace of the current database.

See also default_tablespace.

TimeZone

Sets the time zone for displaying and interpreting time stamps. The default is to use whatever the system environment specifies as the time zone. See Date/Time Keywords in the PostgreSQL documentation.

timezone_abbreviations

Sets the collection of time zone abbreviations that will be accepted by the server for date time input. The default is Default, which is a collection that works in most of the world. Australia and India, and other collections can be defined for a particular installation. Possible values are names of configuration files stored in $GPHOME/share/postgresql/timezonesets/.

To configure LightDB-A Database to use a custom collection of timezones, copy the file that contains the timezone definitions to the directory $GPHOME/share/postgresql/timezonesets/ on the LightDB-A Database coordinator and segment hosts. Then set value of the server configuration parameter timezone_abbreviations to the file. For example, to use a file custom that contains the default timezones and the WIB (Waktu Indonesia Barat) timezone.

1. Copy the file Default from the directory $GPHOME/share/postgresql/timezonesets/ the file custom. Add the WIB timezone information from the file Asia.txt to the custom.
2. Copy the file custom to the directory $GPHOME/share/postgresql/timezonesets/ on the LightDB-A Database coordinator and segment hosts.
3. Set value of the server configuration parameter timezone_abbreviations to custom.
4. Reload the server configuration file (gpstop -u).

track_activities

Enables the collection of information on the currently executing command of each session, along with the time when that command began execution. This parameter is true by default. Only superusers can change this setting. See the pg_stat_activity view.

Note Even when enabled, this information is not visible to all users, only to superusers and the user owning the session being reported on, so it should not represent a security risk.

track_activity_query_size

Sets the maximum length limit for the query text stored in query column of the system catalog view pg_stat_activity. The minimum length is 1024 characters.

track_counts

Collects information about executing commands. Enables the collection of information on the currently executing command of each session, along with the time at which that command began execution.

transaction_isolation

Sets the current transaction’s isolation level.

transaction_read_only

Sets the current transaction’s read-only status.

transform_null_equals

When on, expressions of the form expr = NULL (or NULL = expr) are treated as expr IS NULL, that is, they return true if expr evaluates to the null value, and false otherwise. The correct SQL-spec-compliant behavior of expr = NULL is to always return null (unknown).

unix_socket_directories

Specifies the directory of the UNIX-domain socket on which the server is to listen for connections from client applications. Multiple sockets can be created by listing multiple directories separated by commas.

Important Do not change the value of this parameter. The default location is required for LightDB-A Database utilities.

unix_socket_group

Sets the owning group of the UNIX-domain socket. By default this is an empty string, which uses the default group for the current user.

unix_socket_permissions

Sets the access permissions of the UNIX-domain socket. UNIX-domain sockets use the usual UNIX file system permission set. Note that for a UNIX-domain socket, only write permission matters.

update_process_title

Enables updating of the process title every time a new SQL command is received by the server. The process title is typically viewed by the ps command.

vacuum_cost_delay

The length of time that the process will sleep when the cost limit has been exceeded. 0 deactivates the cost-based vacuum delay feature.

vacuum_cost_limit

The accumulated cost that will cause the vacuuming process to sleep.

vacuum_cost_page_dirty

The estimated cost charged when vacuum modifies a block that was previously clean. It represents the extra I/O required to flush the dirty block out to disk again.

vacuum_cost_page_hit

The estimated cost for vacuuming a buffer found in the shared buffer cache. It represents the cost to lock the buffer pool, lookup the shared hash table and scan the content of the page.

vacuum_cost_page_miss

The estimated cost for vacuuming a buffer that has to be read from disk. This represents the effort to lock the buffer pool, lookup the shared hash table, read the desired block in from the disk and scan its content.

vacuum_freeze_min_age

Specifies the cutoff age (in transactions) that VACUUM should use to decide whether to replace transaction IDs with FrozenXID while scanning a table.

For information about VACUUM and transaction ID management, see “Managing Data” in the LightDB-A Database Administrator Guide and the PostgreSQL documentation.

validate_previous_free_tid

Enables a test that validates the free tuple ID (TID) list. The list is maintained and used by LightDB-A Database. LightDB-A Database determines the validity of the free TID list by ensuring the previous free TID of the current free tuple is a valid free tuple. The default value is true, enable the test.

If LightDB-A Database detects a corruption in the free TID list, the free TID list is rebuilt, a warning is logged, and a warning is returned by queries for which the check failed. LightDB-A Database attempts to run the queries.

Note If a warning is returned, please contact VMware Support.

verify_gpfdists_cert

When a LightDB-A Database external table is defined with the gpfdists protocol to use SSL security, this parameter controls whether SSL certificate authentication is enabled.

Regardless of the setting of this server configuration parameter, LightDB-A Database always encrypts data that you read from or write to an external table that specifies the gpfdists protocol.

The default is true, SSL authentication is enabled when LightDB-A Database communicates with the gpfdist utility to either read data from or write data to an external data source.

The value false deactivates SSL certificate authentication. These SSL exceptions are ignored:

• The self-signed SSL certificate that is used by gpfdist is not trusted by LightDB-A Database.
• The host name contained in the SSL certificate does not match the host name that is running gpfdist.

You can set the value to false to deactivate authentication when testing the communication between the LightDB-A Database external table and the gpfdist utility that is serving the external data.

Caution Deactivating SSL certificate authentication exposes a security risk by not validating the gpfdists SSL certificate.

For information about the gpfdists protocol, see gpfdists:// Protocol. For information about running the gpfdist utility, see gpfdist.

vmem_process_interrupt

Enables checking for interrupts before reserving vmem memory for a query during LightDB-A Database query execution. Before reserving further vmem for a query, check if the current session for the query has a pending query cancellation or other pending interrupts. This ensures more responsive interrupt processing, including query cancellation requests. The default is off.

wait_for_replication_threshold

When LightDB-A Database segment mirroring is enabled, specifies the maximum amount of Write-Ahead Logging (WAL)-based records (in KB) written by a transaction on the primary segment instance before the records are written to the mirror segment instance for replication. As the default, LightDB-A Database writes the records to the mirror segment instance when a checkpoint occurs or the wait_for_replication_threshold value is reached.

A value of 0 deactivates the check for the amount of records. The records are written to the mirror segment instance only after a checkpoint occurs.

If you set the value to 0, database performance issues might occur under heavy loads that perform long transactions that do not perform a checkpoint operation.

wal_keep_segments

For LightDB-A Database coordinator mirroring, sets the maximum number of processed WAL segment files that are saved by the by the active LightDB-A Database coordinator if a checkpoint operation occurs.

The segment files are used to synchronize the active coordinator on the standby coordinator.

wal_receiver_status_interval

For LightDB-A Database coordinator mirroring, sets the interval in seconds between walreceiver process status messages that are sent to the active coordinator. Under heavy loads, the time might be longer.

The value of wal_sender_timeout controls the time that the walsender process waits for a walreceiver message.

writable_external_table_bufsize

Size of the buffer that LightDB-A Database uses for network communication, such as the gpfdist utility and external web tables (that use http). Valid units are KB (as in 128KB), MB, GB, and TB. LightDB-A Database stores data in the buffer before writing the data out. For information about gpfdist, see the LightDB-A Database Utility Guide.

xid_stop_limit

The number of transaction IDs prior to the ID where transaction ID wraparound occurs. When this limit is reached, LightDB-A Database stops creating new transactions to avoid data loss due to transaction ID wraparound.

xid_warn_limit

The number of transaction IDs prior to the limit specified by xid_stop_limit. When LightDB-A Database reaches this limit, it issues a warning to perform a VACUUM operation to avoid data loss due to transaction ID wraparound.

xmlbinary

Specifies how binary values are encoded in XML data. For example, when bytea values are converted to XML. The binary data can be converted to either base64 encoding or hexadecimal encoding. The default is base64.

The parameter can be set for a database system, an individual database, or a session.

xmloption

Specifies whether XML data is to be considered as an XML document (document) or XML content fragment (content) for operations that perform implicit parsing and serialization. The default is content.

This parameter affects the validation performed by xml_is_well_formed(). If the value is document, the function checks for a well-formed XML document. If the value is content, the function checks for a well-formed XML content fragment.

Note An XML document that contains a document type declaration (DTD) is not considered a valid XML content fragment. If xmloption set to content, XML that contains a DTD is not considered valid XML.

To cast a character string that contains a DTD to the xml data type, use the xmlparse function with the document keyword, or change the xmloption value to document.

The parameter can be set for a database system, an individual database, or a session. The SQL command to set this option for a session is also available in LightDB-A Database.

SET XML OPTION { DOCUMENT | CONTENT }