diff options
| -rw-r--r-- | doc/src/sgml/perform.sgml | 33 |
1 files changed, 18 insertions, 15 deletions
diff --git a/doc/src/sgml/perform.sgml b/doc/src/sgml/perform.sgml index 5448913586c..2316df99a5c 100644 --- a/doc/src/sgml/perform.sgml +++ b/doc/src/sgml/perform.sgml @@ -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 < 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> |