+ linkend="maintenance">.
- The collected statistics 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 may 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 .
+ 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 may result
+ in small changes in the planner's estimated costs shown by
+ EXPLAIN. In rare situations, this
+ non-determinism will cause the query optimizer to choose a
+ different query plan between runs of ANALYZE. 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 parameter variable, or on a
- column-by-column basis by setting the per-column
- statistics target with ALTER TABLE ... ALTER COLUMN ... SET
- STATISTICS (see
- ). 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 10, 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 may 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.
+ DEFAULT_STATISTICS_TARGET parameter variable, or
+ on a column-by-column basis by setting the per-column statistics
+ target with ALTER TABLE ... ALTER COLUMN ... SET
+ STATISTICS (see
+ endterm="sql-altertable-title">). 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 10, 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 may 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.
These key words control whether the password is stored
encrypted in the system catalogs. (If neither is specified,
the default behavior is determined by the configuration
- parameter password_encryption.) If the
+ parameter PASSWORD_ENCRYPTION.) If the
presented password string is already in MD5-encrypted format,
then it is stored encrypted as-is, regardless of whether
ENCRYPTED or UNENCRYPTED is specified
- Default value of the <literal>datestyle> run-time
+ Default value of the <varname>DATESTYLE> run-time
parameter. (The use of this environment variable is deprecated.)
$ postmaster -c sort_mem=1234
$ postmaster --sort-mem=1234
- Either form overrides whatever setting might exist for <literal>sort_mem
+ Either form overrides whatever setting might exist for <varname>SORT_MEM
in postgresql.conf. Notice that underscores in parameter
names can be written as either underscore or dash on the command line.
Examples
- Set datestyle to its default value:
+ Set DATESTYLE to its default value:
RESET datestyle;
- Set geqo to its default value:
+ Set GEQO to its default value:
RESET geqo;
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