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Commit4c2d3cc

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Add a caveat pointing out that constraint exclusion doesn't work with
constraints the planner is unable to disprove, hence simple btree-compatibleconditions should be used. We've seen people try to get cute with stufflike date_part(something) = something at least twice now. Even if wewanted to try to teach predtest.c about the properties of date_part,most of the useful variants aren't immutable so nothing could be proved.
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‎doc/src/sgml/ddl.sgml

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