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Create a third option named "partition" for constraint_exclusion, and make it

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the default.  This setting enables constraint exclusion checks only for
appendrel members (ie, inheritance children and UNION ALL arms), which are
the cases in which constraint exclusion is most likely to be useful.  Avoiding
the overhead for simple queries that are unlikely to benefit should bring
the cost down to the point where this is a reasonable default setting.
Per today's discussion.
This commit is contained in:
Tom Lane
2009-01-07 22:40:49 +00:00
parent 12dcf7bb75
commit 445ce15702
7 changed files with 92 additions and 45 deletions
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27
doc/src/sgml/ddl.sgml
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@@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.83 2008/09/08 00:47:40 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.84 2009/01/07 22:40:49 tgl Exp $ -->
<chapter id="ddl">
<title>Data Definition</title>
@@ -2135,7 +2135,7 @@ VALUES ('New York', NULL, NULL, 'NY');
an existing inheritance hierarchy, be careful to grant all the needed
permissions on it.
</para>
<para>
A serious limitation of the inheritance feature is that indexes (including
unique constraints) and foreign key constraints only apply to single
@@ -2394,9 +2394,9 @@ CHECK ( outletID BETWEEN 200 AND 300 )
<listitem>
<para>
Ensure that the <xref linkend="guc-constraint-exclusion">
configuration
parameter is enabled in <filename>postgresql.conf</>. Without
this, queries will not be optimized as desired.
configuration parameter is not disabled in
<filename>postgresql.conf</>.
If it is, queries will not be optimized as desired.
</para>
</listitem>
@@ -2698,7 +2698,7 @@ SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
<para>
You can use the <command>EXPLAIN</> command to show the difference
between a plan with <varname>constraint_exclusion</> on and a plan
with it off. A typical default plan for this type of table setup is:
with it off. A typical unoptimized plan for this type of table setup is:
<programlisting>
SET constraint_exclusion = off;
@@ -2725,7 +2725,7 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
full-table sequential scans, but the point here is that there
is no need to scan the older partitions at all to answer this query.
When we enable constraint exclusion, we get a significantly
reduced plan that will deliver the same answer:
cheaper plan that will deliver the same answer:
<programlisting>
SET constraint_exclusion = on;
@@ -2751,6 +2751,17 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
be helpful in the latter case but not the former.
</para>
<para>
The default (and recommended) setting of
<xref linkend="guc-constraint-exclusion"> is actually neither
<literal>on</> nor <literal>off</>, but an intermediate setting
called <literal>partition</>, which causes the technique to be
applied only to queries that are likely to be working on partitioned
tables. The <literal>on</> setting causes the planner to examine
<literal>CHECK</> constraints in all queries, even simple ones that
are unlikely to benefit.
</para>
</sect2>
<sect2 id="ddl-partitioning-alternatives">
@@ -2817,7 +2828,7 @@ UNION ALL SELECT * FROM measurement_y2008m01;
<sect2 id="ddl-partitioning-caveats">
<title>Caveats</title>
<para>
The following caveats apply to partitioned tables:
<itemizedlist>