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Minor SGML improvements.

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This commit is contained in:
Neil Conway 2005-02-03 07:12:37 +00:00
parent ad476170e9
commit ad893a361d
1 changed files with 18 additions and 15 deletions
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33
doc/src/sgml/perform.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.49 2004/12/23 23:07:38 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.50 2005/02/03 07:12:37 neilc Exp $
-->
<chapter id="performance-tips">
@ -362,12 +362,13 @@ EXPLAIN ANALYZE SELECT * FROM tenk1 t1, tenk2 t2 WHERE t1.unique1 &lt; 50 AND t1
</para>
<para>
One component of the statistics is the total number of entries in each
table and index, as well as the number of disk blocks occupied by each
table and index. This information is kept in the table
<structname>pg_class</structname> in the columns <structfield>reltuples</structfield>
and <structfield>relpages</structfield>. We can look at it
with queries similar to this one:
One component of the statistics is the total number of entries in
each table and index, as well as the number of disk blocks occupied
by each table and index. This information is kept in the table
<link linkend="catalog-pg-class"><structname>pg_class</structname></link>, in
the columns <structfield>reltuples</structfield> and
<structfield>relpages</structfield>. We can look at it with
queries similar to this one:
<screen>
SELECT relname, relkind, reltuples, relpages FROM pg_class WHERE relname LIKE 'tenk1%';
@ -406,14 +407,16 @@ SELECT relname, relkind, reltuples, relpages FROM pg_class WHERE relname LIKE 't
<para>
Most queries retrieve only a fraction of the rows in a table, due
to having <literal>WHERE</> clauses that restrict the rows to be examined.
The planner thus needs to make an estimate of the
<firstterm>selectivity</> of <literal>WHERE</> clauses, that is, the fraction of
rows that match each condition in the <literal>WHERE</> clause. The information
used for this task is stored in the <structname>pg_statistic</structname>
system catalog. Entries in <structname>pg_statistic</structname> are
updated by <command>ANALYZE</> and <command>VACUUM ANALYZE</> commands
and are always approximate even when freshly updated.
to having <literal>WHERE</> clauses that restrict the rows to be
examined. The planner thus needs to make an estimate of the
<firstterm>selectivity</> of <literal>WHERE</> clauses, that is,
the fraction of rows that match each condition in the
<literal>WHERE</> clause. The information used for this task is
stored in the <link linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link>
system catalog. Entries in <structname>pg_statistic</structname>
are updated by the <command>ANALYZE</> and <command>VACUUM
ANALYZE</> commands and are always approximate even when freshly
updated.
</para>
<indexterm>