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Remove overly-pessimistic statement about constraint exclusion.

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As of 9.2, constraint exclusion should work okay with prepared statements:
the planner will try custom plans with actual values of the parameters,
and observe that they are a lot cheaper than the generic plan, and thus
never fall back to using the generic plan.  Noted by Tatsuhito Kasahara.
This commit is contained in:
Tom Lane 2012-08-22 23:55:34 -04:00
parent b33f611682
commit 6e2e12c947
1 changed files with 5 additions and 5 deletions
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10
doc/src/sgml/ddl.sgml
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@ -2975,11 +2975,11 @@ ANALYZE measurement;
<listitem> <listitem>
<para> <para>
Constraint exclusion only works when the query's <literal>WHERE</> Constraint exclusion only works when the query's <literal>WHERE</>
clause contains constants. A parameterized query will not be clause contains constants (or externally supplied parameters).
optimized, since the planner cannot know which partitions the For example, a comparison against a non-immutable function such as
parameter value might select at run time. For the same reason, <function>CURRENT_TIMESTAMP</function> cannot be optimized, since the
<quote>stable</> functions such as <function>CURRENT_DATE</function> planner cannot know which partition the function value might fall
must be avoided. into at run time.
</para> </para>
</listitem> </listitem>