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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"> <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>
<para> <para>
One component of the statistics is the total number of entries in each One component of the statistics is the total number of entries in
table and index, as well as the number of disk blocks occupied by each each table and index, as well as the number of disk blocks occupied
table and index. This information is kept in the table by each table and index. This information is kept in the table
<structname>pg_class</structname> in the columns <structfield>reltuples</structfield> <link linkend="catalog-pg-class"><structname>pg_class</structname></link>, in
and <structfield>relpages</structfield>. We can look at it the columns <structfield>reltuples</structfield> and
with queries similar to this one: <structfield>relpages</structfield>. We can look at it with
queries similar to this one:
<screen> <screen>
SELECT relname, relkind, reltuples, relpages FROM pg_class WHERE relname LIKE 'tenk1%'; 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> <para>
Most queries retrieve only a fraction of the rows in a table, due 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. to having <literal>WHERE</> clauses that restrict the rows to be
The planner thus needs to make an estimate of the examined. The planner thus needs to make an estimate of the
<firstterm>selectivity</> of <literal>WHERE</> clauses, that is, the fraction of <firstterm>selectivity</> of <literal>WHERE</> clauses, that is,
rows that match each condition in the <literal>WHERE</> clause. The information the fraction of rows that match each condition in the
used for this task is stored in the <structname>pg_statistic</structname> <literal>WHERE</> clause. The information used for this task is
system catalog. Entries in <structname>pg_statistic</structname> are stored in the <link linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link>
updated by <command>ANALYZE</> and <command>VACUUM ANALYZE</> commands system catalog. Entries in <structname>pg_statistic</structname>
and are always approximate even when freshly updated. are updated by the <command>ANALYZE</> and <command>VACUUM
ANALYZE</> commands and are always approximate even when freshly
updated.
</para> </para>
<indexterm> <indexterm>