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| -rw-r--r-- | doc/src/sgml/ddl.sgml | 22 |
1 files changed, 17 insertions, 5 deletions
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml index 33749f5f200..5525ebb5bd2 100644 --- a/doc/src/sgml/ddl.sgml +++ b/doc/src/sgml/ddl.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.75 2007/05/15 19:43:50 neilc Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.76 2007/06/20 23:11:38 tgl Exp $ --> <chapter id="ddl"> <title>Data Definition</title> @@ -2778,7 +2778,7 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01'; <para> Constraint exclusion only works when the query's <literal>WHERE</> clause contains constants. A parameterized query will not be - optimized, since the planner cannot know what partitions the + optimized, since the planner cannot know which partitions the parameter value might select at run time. For the same reason, <quote>stable</> functions such as <function>CURRENT_DATE</function> must be avoided. @@ -2787,9 +2787,21 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01'; <listitem> <para> - All constraints on all partitions of the master table are considered for - constraint exclusion, so large numbers of partitions are likely to - increase query planning time considerably. + Keep the partitioning constraints simple, else the planner may not be + able to prove that partitions don't need to be visited. Use simple + equality conditions for list partitioning, or simple + range tests for range partitioning, as illustrated in the preceding + examples. A good rule of thumb is that partitioning constraints should + contain only comparisons of the partitioning column(s) to constants + using btree-indexable operators. + </para> + </listitem> + + <listitem> + <para> + All constraints on all partitions of the master table are examined + during constraint exclusion, so large numbers of partitions are likely + to increase query planning time considerably. </para> </listitem> |