ANALYZE — collect statistics about a database
ANALYZE [ (option[, ...] ) ] [table_and_columns[, ...] ] ANALYZE [ VERBOSE ] [table_and_columns[, ...] ] whereoptioncan be one of: VERBOSE [boolean] SKIP_LOCKED [boolean] andtable_and_columnsis:table_name[ (column_name[, ...] ) ]
ANALYZE collects statistics about the contents
of tables in the database, and stores the results in the pg_statistic
system catalog. Subsequently, the query planner uses these
statistics to help determine the most efficient execution plans for
queries.
Without a table_and_columns
list, ANALYZE processes every table and
in the current database that the current user has permission to analyze.
With a list, ANALYZE processes only those table(s).
It is further possible to give a list of column names for a table,
in which case only the statistics for those columns are collected.
When the option list is surrounded by parentheses, the options can be written in any order. The parenthesized syntax was added in LightDB 11; the unparenthesized syntax is deprecated.
VERBOSEEnables display of progress messages.
SKIP_LOCKED
Specifies that ANALYZE should not wait for any
conflicting locks to be released when beginning work on a relation:
if a relation cannot be locked immediately without waiting, the relation
is skipped. Note that even with this option, ANALYZE
may still block when opening the relation's indexes or when acquiring
sample rows from partitions, table inheritance children, and some
types of foreign tables. Also, while ANALYZE
ordinarily processes all partitions of specified partitioned tables,
this option will cause ANALYZE to skip all
partitions if there is a conflicting lock on the partitioned table.
boolean
Specifies whether the selected option should be turned on or off.
You can write TRUE, ON, or
1 to enable the option, and FALSE,
OFF, or 0 to disable it. The
boolean value can also
be omitted, in which case TRUE is assumed.
table_nameThe name (possibly schema-qualified) of a specific table to analyze. If omitted, all regular tables, partitioned tables in the current database are analyzed (but not foreign tables). If the specified table is a partitioned table, both the inheritance statistics of the partitioned table as a whole and statistics of the individual partitions are updated.
column_nameThe name of a specific column to analyze. Defaults to all columns.
When VERBOSE is specified, ANALYZE emits
progress messages to indicate which table is currently being
processed. Various statistics about the tables are printed as well.
To analyze a table, one must ordinarily be the table's owner or a
superuser. However, database owners are allowed to
analyze all tables in their databases, except shared catalogs.
(The restriction for shared catalogs means that a true database-wide
ANALYZE can only be performed by a superuser.)
ANALYZE will skip over any tables that the calling user
does not have permission to analyze.
Foreign tables are analyzed only when explicitly selected. Not all
foreign data wrappers support ANALYZE. If the table's
wrapper does not support ANALYZE, the command prints a
warning and does nothing.
In the default LightDB configuration,
the autovacuum daemon (see Section 23.1.6)
takes care of automatic analyzing of tables when they are first loaded
with data, and as they change throughout regular operation.
When autovacuum is disabled,
it is a good idea to run ANALYZE periodically, or
just after making major changes in the contents of a table. Accurate
statistics will help the planner to choose the most appropriate query
plan, and thereby improve the speed of query processing. A common
strategy for read-mostly databases is to run VACUUM
and ANALYZE once a day during a low-usage time of day.
(This will not be sufficient if there is heavy update activity.)
ANALYZE
requires only a read lock on the target table, so it can run in
parallel with other activity on the table.
The statistics collected by ANALYZE usually
include a list of some of the most common values in each column and
a histogram showing the approximate data distribution in each
column. One or both of these can be omitted if
ANALYZE deems them uninteresting (for example,
in a unique-key column, there are no common values) or if the
column data type does not support the appropriate operators. There
is more information about the statistics in Chapter 23.
For large tables, ANALYZE takes a random sample
of the table contents, rather than examining every row. This
allows even very large tables to be analyzed in a small amount of
time. Note, however, that the statistics are only approximate, and
will change slightly each time ANALYZE is run,
even if the actual table contents did not change. This might result
in small changes in the planner's estimated costs shown by
EXPLAIN.
In rare situations, this non-determinism will cause the planner's
choices of query plans to change after ANALYZE is run.
To avoid this, raise the amount of statistics collected by
ANALYZE, as described below.
The extent of analysis can be controlled by adjusting the
default_statistics_target configuration variable, or
on a column-by-column basis by setting the per-column statistics
target with ALTER TABLE ... ALTER COLUMN ... SET
STATISTICS (see ALTER TABLE).
The target value sets the
maximum number of entries in the most-common-value list and the
maximum number of bins in the histogram. The default target value
is 100, but this can be adjusted up or down to trade off accuracy of
planner estimates against the time taken for
ANALYZE and the amount of space occupied in
pg_statistic. In particular, setting the
statistics target to zero disables collection of statistics for
that column. It might be useful to do that for columns that are
never used as part of the WHERE, GROUP BY,
or ORDER BY clauses of queries, since the planner will
have no use for statistics on such columns.
The largest statistics target among the columns being analyzed determines
the number of table rows sampled to prepare the statistics. Increasing
the target causes a proportional increase in the time and space needed
to do ANALYZE.
One of the values estimated by ANALYZE is the number of
distinct values that appear in each column. Because only a subset of the
rows are examined, this estimate can sometimes be quite inaccurate, even
with the largest possible statistics target. If this inaccuracy leads to
bad query plans, a more accurate value can be determined manually and then
installed with
ALTER TABLE ... ALTER COLUMN ... SET (n_distinct = ...)
(see ALTER TABLE).
If the table being analyzed has inheritance children,
ANALYZE gathers two sets of statistics: one on the rows
of the parent table only, and a second including rows of both the parent
table and all of its children. This second set of statistics is needed when
planning queries that process the inheritance tree as a whole. The child
tables themselves are not individually analyzed in this case.
The autovacuum daemon, however, will only consider inserts or
updates on the parent table itself when deciding whether to trigger an
automatic analyze for that table. If that table is rarely inserted into
or updated, the inheritance statistics will not be up to date unless you
run ANALYZE manually.
For partitioned tables, ANALYZE gathers statistics by
sampling rows from all partitions; in addition, it will recurse into each
partition and update its statistics. Each leaf partition is analyzed only
once, even with multi-level partitioning. No statistics are collected for
only the parent table (without data from its partitions), because with
partitioning it's guaranteed to be empty.
The autovacuum daemon does not process partitioned tables, nor does it
process inheritance parents if only the children are ever modified.
It is usually necessary to periodically run a manual
ANALYZE to keep the statistics of the table hierarchy
up to date.
If any child tables or partitions are foreign tables whose foreign
data wrappers do not support ANALYZE, those tables are
ignored while gathering inheritance statistics.
If the table being analyzed is completely empty, ANALYZE
will not record new statistics for that table. Any existing statistics
will be retained.
There is no ANALYZE statement in the SQL standard.