ANALYZE

ANALYZE

7

SQL - Language Statements

ANALYZE

collect statistics about a database

ANALYZE \[ (

option

\[, \...\] ) \] \[

table\_and\_columns

\[, \...\] \] ANALYZE \[ VERBOSE \] \[

table\_and\_columns

\[, \...\] \]

where

option

can be one of:

VERBOSE \[

boolean

\] SKIP\_LOCKED \[

boolean

\]

and

table\_and\_columns

is:

table\_name

\[ (

column\_name

\[, \...\] ) \]

Description
===========

`ANALYZE` collects statistics about the contents of tables in the
database, and stores the results in the
[pg\_statistic](#catalog-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
materialized view 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 PostgreSQL
11; the unparenthesized syntax is deprecated.

Parameters
==========

`VERBOSE`

:   Enables 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\_name

:   The name (possibly schema-qualified) of a specific table to analyze.
    If omitted, all regular tables, partitioned tables, and materialized
    views 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\_name

:   The name of a specific column to analyze. Defaults to all columns.

Outputs
=======

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.

Notes
=====

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 PostgreSQL configuration, the autovacuum daemon (see
[???](#autovacuum)) 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`](#sql-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 [???](#maintenance).

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`](#sql-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
[???](#guc-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`](#sql-altertable). 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 = ...)`](#sql-altertable).

If the table being analyzed has one or more children, `ANALYZE` will
gather statistics twice: once on the rows of the parent table only, and
a second time on the rows of the parent table with all of its children.
This second set of statistics is needed when planning queries that
traverse the entire inheritance tree. 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.

By contrast, 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 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.

Each backend running `ANALYZE` will report its progress in the
pg\_stat\_progress\_analyze view. See [???](#analyze-progress-reporting)
for details.

Compatibility
=============

There is no `ANALYZE` statement in the SQL standard.

See Also
========
