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Commit731ab0a

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Doc: improve documentation about run-time pruning's effects on EXPLAIN.
Tatsuo Ishii complained that this para wasn't very intelligible.Try to make it better.Discussion:https://postgr.es/m/20191207.200500.989741087350666720.t-ishii@sraoss.co.jp
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‎doc/src/sgml/perform.sgml

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@@ -896,15 +896,16 @@ EXPLAIN ANALYZE SELECT * FROM tenk1 WHERE unique1 < 100 AND unique2 > 9000
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</para>
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<para>
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Generally, the <command>EXPLAIN</command> output will display details for
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every plan node which was generated by the query planner. However, there
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are cases where the executor is able to determine that certain nodes are
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not required; currently, the only node type to support this is the
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<literal>Append</literal> node. This node type has the ability to discard
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subnodes which it is able to determine won't contain any records required
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by the query. It is possible to determine that nodes have been removed in
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this way by the presence of a "Subplans Removed" property in the
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<command>EXPLAIN</command> output.
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Normally, <command>EXPLAIN</command> will display every plan node
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created by the planner. However, there are cases where the executor
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can determine that certain nodes need not be executed because they
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cannot produce any rows, based on parameter values that were not
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available at planning time. (Currently this can only happen for child
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nodes of an Append node that is scanning a partitioned
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table.) When this happens, those plan nodes are omitted from
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the <command>EXPLAIN</command> output and a <literal>Subplans
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Removed: <replaceable>N</replaceable></literal> annotation appears
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instead.
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</para>
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</sect2>
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