-<!-- $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>
<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.
<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>