The ltstattuple
module provides various functions to
obtain tuple-level statistics.
Because these functions return detailed page-level information, access is
restricted by default. By default, only the
role pg_stat_scan_tables
has EXECUTE
privilege. Superusers of course bypass this restriction. After the
extension has been installed, users may issue GRANT
commands to change the privileges on the functions to allow others to
execute them. However, it might be preferable to add those users to
the pg_stat_scan_tables
role instead.
pgstattuple(regclass) returns record
pgstattuple
returns a relation's physical length,
percentage of “dead” tuples, and other info. This may help users
to determine whether vacuum is necessary or not. The argument is the
target relation's name (optionally schema-qualified) or OID.
For example:
test=> SELECT * FROM pgstattuple('pg_catalog.pg_proc'); -[ RECORD 1 ]------+------- table_len | 458752 tuple_count | 1470 tuple_len | 438896 tuple_percent | 95.67 dead_tuple_count | 11 dead_tuple_len | 3157 dead_tuple_percent | 0.69 free_space | 8932 free_percent | 1.95
The output columns are described in Table F.28.
Table F.28. pgstattuple
Output Columns
Column | Type | Description |
---|---|---|
table_len | bigint | Physical relation length in bytes |
tuple_count | bigint | Number of live tuples |
tuple_len | bigint | Total length of live tuples in bytes |
tuple_percent | float8 | Percentage of live tuples |
dead_tuple_count | bigint | Number of dead tuples |
dead_tuple_len | bigint | Total length of dead tuples in bytes |
dead_tuple_percent | float8 | Percentage of dead tuples |
free_space | bigint | Total free space in bytes |
free_percent | float8 | Percentage of free space |
The table_len
will always be greater than the sum
of the tuple_len
, dead_tuple_len
and free_space
. The difference is accounted for by
fixed page overhead, the per-page table of pointers to tuples, and
padding to ensure that tuples are correctly aligned.
pgstattuple
acquires only a read lock on the
relation. So the results do not reflect an instantaneous snapshot;
concurrent updates will affect them.
pgstattuple
judges a tuple is “dead” if
HeapTupleSatisfiesDirty
returns false.
pgstattuple(text) returns record
This is the same as pgstattuple(regclass)
, except
that the target relation is specified as TEXT. This function is kept
because of backward-compatibility so far, and will be deprecated in
some future release.
pgstatindex(regclass) returns record
pgstatindex
returns a record showing information
about a B-tree index. For example:
test=> SELECT * FROM pgstatindex('pg_cast_oid_index'); -[ RECORD 1 ]------+------ version | 2 tree_level | 0 index_size | 16384 root_block_no | 1 internal_pages | 0 leaf_pages | 1 empty_pages | 0 deleted_pages | 0 avg_leaf_density | 54.27 leaf_fragmentation | 0
The output columns are:
Column | Type | Description |
---|---|---|
version | integer | B-tree version number |
tree_level | integer | Tree level of the root page |
index_size | bigint | Total index size in bytes |
root_block_no | bigint | Location of root page (zero if none) |
internal_pages | bigint | Number of “internal” (upper-level) pages |
leaf_pages | bigint | Number of leaf pages |
empty_pages | bigint | Number of empty pages |
deleted_pages | bigint | Number of deleted pages |
avg_leaf_density | float8 | Average density of leaf pages |
leaf_fragmentation | float8 | Leaf page fragmentation |
The reported index_size
will normally correspond to one more
page than is accounted for by internal_pages + leaf_pages +
empty_pages + deleted_pages
, because it also includes the
index's metapage.
As with pgstattuple
, the results are accumulated
page-by-page, and should not be expected to represent an
instantaneous snapshot of the whole index.
pgstatindex(text) returns record
This is the same as pgstatindex(regclass)
, except
that the target index is specified as TEXT. This function is kept
because of backward-compatibility so far, and will be deprecated in
some future release.
pgstatginindex(regclass) returns record
pgstatginindex
returns a record showing information
about a GIN index. For example:
test=> SELECT * FROM pgstatginindex('test_gin_index'); -[ RECORD 1 ]--+-- version | 1 pending_pages | 0 pending_tuples | 0
The output columns are:
Column | Type | Description |
---|---|---|
version | integer | GIN version number |
pending_pages | integer | Number of pages in the pending list |
pending_tuples | bigint | Number of tuples in the pending list |
pgstathashindex(regclass) returns record
pgstathashindex
returns a record showing information
about a HASH index. For example:
test=> select * from pgstathashindex('con_hash_index'); -[ RECORD 1 ]--+----------------- version | 4 bucket_pages | 33081 overflow_pages | 0 bitmap_pages | 1 unused_pages | 32455 live_items | 10204006 dead_items | 0 free_percent | 61.8005949100872
The output columns are:
Column | Type | Description |
---|---|---|
version | integer | HASH version number |
bucket_pages | bigint | Number of bucket pages |
overflow_pages | bigint | Number of overflow pages |
bitmap_pages | bigint | Number of bitmap pages |
unused_pages | bigint | Number of unused pages |
live_items | bigint | Number of live tuples |
dead_tuples | bigint | Number of dead tuples |
free_percent | float | Percentage of free space |
pg_relpages(regclass) returns bigint
pg_relpages
returns the number of pages in the
relation.
pg_relpages(text) returns bigint
This is the same as pg_relpages(regclass)
, except
that the target relation is specified as TEXT. This function is kept
because of backward-compatibility so far, and will be deprecated in
some future release.
pgstattuple_approx(regclass) returns record
pgstattuple_approx
is a faster alternative to
pgstattuple
that returns approximate results.
The argument is the target relation's name or OID.
For example:
test=> SELECT * FROM pgstattuple_approx('pg_catalog.pg_proc'::regclass); -[ RECORD 1 ]--------+------- table_len | 573440 scanned_percent | 2 approx_tuple_count | 2740 approx_tuple_len | 561210 approx_tuple_percent | 97.87 dead_tuple_count | 0 dead_tuple_len | 0 dead_tuple_percent | 0 approx_free_space | 11996 approx_free_percent | 2.09
The output columns are described in Table F.29.
Whereas pgstattuple
always performs a
full-table scan and returns an exact count of live and dead tuples
(and their sizes) and free space, pgstattuple_approx
tries to avoid the full-table scan and returns exact dead tuple
statistics along with an approximation of the number and
size of live tuples and free space.
It does this by skipping pages that have only visible tuples according to the visibility map (if a page has the corresponding VM bit set, then it is assumed to contain no dead tuples). For such pages, it derives the free space value from the free space map, and assumes that the rest of the space on the page is taken up by live tuples.
For pages that cannot be skipped, it scans each tuple, recording its presence and size in the appropriate counters, and adding up the free space on the page. At the end, it estimates the total number of live tuples based on the number of pages and tuples scanned (in the same way that VACUUM estimates pg_class.reltuples).
Table F.29. pgstattuple_approx
Output Columns
Column | Type | Description |
---|---|---|
table_len | bigint | Physical relation length in bytes (exact) |
scanned_percent | float8 | Percentage of table scanned |
approx_tuple_count | bigint | Number of live tuples (estimated) |
approx_tuple_len | bigint | Total length of live tuples in bytes (estimated) |
approx_tuple_percent | float8 | Percentage of live tuples |
dead_tuple_count | bigint | Number of dead tuples (exact) |
dead_tuple_len | bigint | Total length of dead tuples in bytes (exact) |
dead_tuple_percent | float8 | Percentage of dead tuples |
approx_free_space | bigint | Total free space in bytes (estimated) |
approx_free_percent | float8 | Percentage of free space |
In the above output, the free space figures may not match the
pgstattuple
output exactly, because the free
space map gives us an exact figure, but is not guaranteed to be
accurate to the byte.
Tatsuo Ishii, Satoshi Nagayasu and Abhijit Menon-Sen