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Docs: explain contrib/pg_stat_statements' handling of GC failure.
Failure to perform garbage collection now has a user-visible effect, so explain that and explain that reducing pgss_max is the way to prevent it. Per gripe from Andrew Dunstan.
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@ -270,7 +270,7 @@
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</para>
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<para>
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Consumers of <literal>pg_stat_statements</> may wish to use
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Consumers of <structname>pg_stat_statements</> may wish to use
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<structfield>queryid</> (perhaps in combination with
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<structfield>dbid</> and <structfield>userid</>) as a more stable
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and reliable identifier for each entry than its query text.
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@ -280,7 +280,7 @@
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post-parse-analysis tree, its value is a function of, among other
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things, the internal object identifiers appearing in this representation.
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This has some counterintuitive implications. For example,
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<literal>pg_stat_statements</> will consider two apparently-identical
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<filename>pg_stat_statements</> will consider two apparently-identical
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queries to be distinct, if they reference a table that was dropped
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and recreated between the executions of the two queries.
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The hashing process is also sensitive to differences in
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@ -300,6 +300,20 @@
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not be a useful identifier for accumulating costs across a set of logical
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replicas. If in doubt, direct testing is recommended.
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</para>
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<para>
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The representative query texts are kept in an external disk file, and do
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not consume shared memory. Therefore, even very lengthy query texts can
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be stored successfully. However, if many long query texts are
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accumulated, the external file might grow unmanageably large. As a
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recovery method if that happens, <filename>pg_stat_statements</> may
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choose to discard the query texts, whereupon all existing entries in
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the <structname>pg_stat_statements</> view will show
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null <structfield>query</> fields, though the statistics associated with
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each <structfield>queryid</> are preserved. If this happens, consider
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reducing <varname>pg_stat_statements.max</varname> to prevent
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recurrences.
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</para>
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</sect2>
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<sect2>
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