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Commitade9158

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Remove the old advice to keep from_collapse_limit less than geqo_threshold,
instead just pointing out that a larger value may trigger use of GEQO.Per Robert Haas.In passing, do a bit of wordsmithing on the Genetic Query Optimizer section.
1 parent7c8d7a2 commitade9158

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‎doc/src/sgml/config.sgml‎

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<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.216 2009/04/27 16:27:35 momjian Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.217 2009/06/02 17:37:55 tgl Exp $ -->
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<chapter Id="runtime-config">
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<title>Server Configuration</title>
@@ -2025,6 +2025,16 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
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<sect2 id="runtime-config-query-geqo">
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<title>Genetic Query Optimizer</title>
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<para>
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The genetic query optimizer (GEQO) is an algorithm that does query
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planning using heuristic searching. This reduces planning time for
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complex queries (those joining many relations), at the cost of producing
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plans that are sometimes inferior to those found by the normal
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exhaustive-search algorithm. Also, GEQO's searching is randomized and
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therefore its plans may vary nondeterministically.
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For more information see <xref linkend="geqo">.
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</para>
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<variablelist>
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<varlistentry id="guc-geqo" xreflabel="geqo">
@@ -2041,11 +2051,10 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
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<term><varname>geqo</varname> (<type>boolean</type>)</term>
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<listitem>
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<para>
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Enables or disables genetic query optimization, which is an
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algorithm that attempts to do query planning without
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exhaustive searching. This is on by default. The
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<varname>geqo_threshold</varname> variable provides a more
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granular way to disable GEQO for certain classes of queries.
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Enables or disables genetic query optimization.
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This is on by default. It is usually best not to turn it off in
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production; the <varname>geqo_threshold</varname> variable provides a
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more granular way to control use of GEQO.
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</para>
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</listitem>
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</varlistentry>
@@ -2075,8 +2084,8 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
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</indexterm>
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<listitem>
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<para>
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Controls the tradeoff between planning time and query plan
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efficiency in GEQO. This variable must be an integer in the
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Controls the trade-off between planning time and query plan
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quality in GEQO. This variable must be an integer in the
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range from 1 to 10. The default value is five. Larger values
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increase the time spent doing query planning, but also
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increase the likelihood that an efficient query plan will be
@@ -2100,11 +2109,11 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
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</indexterm>
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<listitem>
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<para>
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Controls the pool size used by GEQO. The pool size is the
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Controls the pool size used by GEQO, that is the
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number of individuals in the genetic population. It must be
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at least two, and useful values are typically 100 to 1000. If
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it is set to zero (the default setting) then a suitable
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default is chosen based on <varname>geqo_effort</varname> and
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value is chosen based on <varname>geqo_effort</varname> and
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the number of tables in the query.
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</para>
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</listitem>
@@ -2117,11 +2126,11 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
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</indexterm>
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<listitem>
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<para>
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Controls the number of generations used by GEQO. Generations
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specifiesthe number of iterations of the algorithm. It must
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Controls the number of generations used by GEQO, that is
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the number of iterations of the algorithm. It must
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be at least one, and useful values are in the same range as
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the pool size. If it is set to zero (the default setting)
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then a suitabledefault is chosen based on
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then a suitablevalue is chosen based on
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<varname>geqo_pool_size</varname>.
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</para>
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</listitem>
@@ -2252,10 +2261,15 @@ SELECT * FROM parent WHERE key = 2400;
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The planner will merge sub-queries into upper queries if the
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resulting <literal>FROM</literal> list would have no more than
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this many items. Smaller values reduce planning time but might
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yield inferior query plans. The default is eight. It is usually
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wise to keep this less than <xref linkend="guc-geqo-threshold">.
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yield inferior query plans. The default is eight.
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For more information see <xref linkend="explicit-joins">.
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</para>
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<para>
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Setting this value to <xref linkend="guc-geqo-threshold"> or more
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may trigger use of the GEQO planner, resulting in nondeterministic
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plans. See <xref linkend="runtime-config-query-geqo">.
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</para>
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</listitem>
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</varlistentry>
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@@ -2285,6 +2299,12 @@ SELECT * FROM parent WHERE key = 2400;
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order they desire explicitly.
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For more information see <xref linkend="explicit-joins">.
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</para>
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<para>
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Setting this value to <xref linkend="guc-geqo-threshold"> or more
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may trigger use of the GEQO planner, resulting in nondeterministic
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plans. See <xref linkend="runtime-config-query-geqo">.
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</para>
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</listitem>
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</varlistentry>
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