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This patch makes a few minor improvements to the docs: make the

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<varname> conventions more consistent, and improve the ANALYZE ref page.

Neil Conway
This commit is contained in:
Bruce Momjian
2003-09-11 17:31:45 +00:00
parent 64a7b58aa0
commit 8e27be4310
6 changed files with 57 additions and 50 deletions
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83
doc/src/sgml/ref/analyze.sgml
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@@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/analyze.sgml,v 1.14 2003/09/09 18:28:52 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/analyze.sgml,v 1.15 2003/09/11 17:31:45 momjian Exp $
PostgreSQL documentation
-->
@@ -28,10 +28,10 @@ ANALYZE [ VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> [ (<rep
<title>Description</title>
<para>
<command>ANALYZE</command> collects statistics about the contents of
tables in the database, and stores the results in
the system table <literal>pg_statistic</literal>. Subsequently,
the query planner uses the statistics to help determine the most efficient
<command>ANALYZE</command> collects statistics about the contents
of tables in the database, and stores the results in the system
table <literal>pg_statistic</literal>. Subsequently, the query
planner uses these statistics to help determine the most efficient
execution plans for queries.
</para>
@@ -90,49 +90,56 @@ ANALYZE [ VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> [ (<rep
</para>
<para>
Unlike <command>VACUUM FULL</command>,
<command>ANALYZE</command> requires
only a read lock on the target table, so it can run in parallel with
other activity on the table.
Unlike <command>VACUUM FULL</command>, <command>ANALYZE</command>
requires only a read lock on the target table, so it can run in
parallel with other activity on the table.
</para>
<para>
For large tables, <command>ANALYZE</command> 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 <command>ANALYZE</command> 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 <command>EXPLAIN</command>.
The statistics collected by <command>ANALYZE</command> 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
<command>ANALYZE</command> 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 <xref
linkend="maintenance">.
</para>
<para>
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
<command>ANALYZE</command> 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 <xref linkend="maintenance">.
For large tables, <command>ANALYZE</command> 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 <command>ANALYZE</command> 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
<command>EXPLAIN</command>. In rare situations, this
non-determinism will cause the query optimizer to choose a
different query plan between runs of <command>ANALYZE</command>. To
avoid this, raise the amount of statistics collected by
<command>ANALYZE</command>, as described below.
</para>
<para>
The extent of analysis can be controlled by adjusting the
<literal>default_statistics_target</> parameter variable, or on a
column-by-column basis by setting the per-column
statistics target with <command>ALTER TABLE ... ALTER COLUMN ... SET
STATISTICS</command> (see
<xref linkend="sql-altertable" 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
<command>ANALYZE</command> and the amount of space occupied
in <literal>pg_statistic</literal>.
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 <literal>WHERE</>, <literal>GROUP BY</>, or <literal>ORDER BY</> clauses of
queries, since the planner will have no use for statistics on such columns.
<varname>DEFAULT_STATISTICS_TARGET</varname> parameter variable, or
on a column-by-column basis by setting the per-column statistics
target with <command>ALTER TABLE ... ALTER COLUMN ... SET
STATISTICS</command> (see <xref linkend="sql-altertable"
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
<command>ANALYZE</command> and the amount of space occupied in
<literal>pg_statistic</literal>. 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 <literal>WHERE</>, <literal>GROUP BY</>,
or <literal>ORDER BY</> clauses of queries, since the planner will
have no use for statistics on such columns.
</para>
<para>