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Commit3256722

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Doc: mention executor memory usage for enable_partitionwise* GUCs
Prior to this commit, the docs for enable_partitionwise_aggregate andenable_partitionwise_join mentioned the additional overheads enablingthese causes for the query planner, but they mentioned nothing about thepossible surge in work_mem-consuming executor nodes that could end up inthe final plan. Dimitrios reported the OOM killer intervened on hisquery as a result of using enable_partitionwise_aggregate=on.Here we adjust the docs to mention the possible increase in the number ofwork_mem-consuming executor nodes that can appear in the final plan as aresult of enabling these GUCs.Reported-by: Dimitrios ApostolouReviewed-by: Ashutosh BapatDiscussion:https://postgr.es/m/3603c380-d094-136e-e333-610914fb3e80%40gmx.netDiscussion:https://postgr.es/m/CAApHDvoZ0_yqwPFEpb6h261L76BUpmh5GxBQq0LeRzQ5Jh3zzg@mail.gmail.comBackpatch-through: 12, oldest supported version
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‎doc/src/sgml/config.sgml

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@@ -5567,9 +5567,13 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
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joining the matching partitions. Partitionwise join currently applies
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only when the join conditions include all the partition keys, which
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must be of the same data type and have one-to-one matching sets of
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child partitions. Because partitionwise join planning can use
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significantly more CPU time and memory during planning, the default is
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<literal>off</literal>.
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child partitions. With this setting enabled, the number of nodes
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whose memory usage is restricted by <varname>work_mem</varname>
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appearing in the final plan can increase linearly according to the
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number of partitions being scanned. This can result in a large
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increase in overall memory consumption during the execution of the
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query. Query planning also becomes significantly more expensive in
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terms of memory and CPU. The default value is <literal>off</literal>.
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</para>
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</listitem>
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</varlistentry>
@@ -5587,9 +5591,13 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
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tables to be performed separately for each partition. If the
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<literal>GROUP BY</literal> clause does not include the partition
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keys, only partial aggregation can be performed on a per-partition
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basis, and finalization must be performed later. Because
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partitionwise grouping or aggregation can use significantly more CPU
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time and memory during planning, the default is
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basis, and finalization must be performed later. With this setting
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enabled, the number of nodes whose memory usage is restricted by
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<varname>work_mem</varname> appearing in the final plan can increase
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linearly according to the number of partitions being scanned. This
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can result in a large increase in overall memory consumption during
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the execution of the query. Query planning also becomes significantly
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more expensive in terms of memory and CPU. The default value is
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<literal>off</literal>.
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</para>
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</listitem>

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