ANALYZE - collect statistics about a database
ANALYZE [ (
option[, ...] ) ] [
table_and_columns[, ...] ] ANALYZE [ VERBOSE ] [
table_and_columns[, ...] ] where
optioncan be one of: VERBOSE and
column_name[, ...] ) ]
collects statistics about the contents
of tables in the database, and stores the results in the
system catalog. Subsequently, the query planner uses these
statistics to help determine the most efficient execution plans for
processes every table and materialized view
in the current database that the current user has permission to analyze.
With a list,
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.
Enables display of progress messages.
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.
The name of a specific column to analyze. Defaults to all columns.
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
can only be performed by a superuser.)
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
. If the table's
wrapper does not support
, the command prints a
warning and does nothing.
In the default
the autovacuum daemon (see
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
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
once a day during a low-usage time of day.
(This will not be sufficient if there is heavy update activity.)
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
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
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
For large tables,
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
even if the actual table contents did not change. This might result
in small changes in the planner's estimated costs shown by
In rare situations, this non-determinism will cause the planner's
choices of query plans to change after
To avoid this, raise the amount of statistics collected by
, as described below.
The extent of analysis can be controlled by adjusting the
configuration variable, or
on a column-by-column basis by setting the per-column statistics
ALTER TABLE ... ALTER COLUMN ... SET
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
and the amount of space occupied in
. 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
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
One of the values estimated by
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
ALTER TABLE ... ALTER COLUMN ... SET (n_distinct = ...)
If the table being analyzed has one or more children,
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
For partitioned tables,
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,
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
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
, those tables are
ignored while gathering inheritance statistics.
If the table being analyzed is completely empty,
will not record new statistics for that table. Any existing statistics
will be retained.
There is no
statement in the SQL standard.