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Docs: explain contrib/pg_stat_statements' handling of GC failure.

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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.
This commit is contained in:
Tom Lane
2015-10-05 12:44:12 -04:00
parent ffdf2a20f1
commit e96b697adb
1 changed files with 16 additions and 2 deletions
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18
doc/src/sgml/pgstatstatements.sgml
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@ -242,7 +242,7 @@
</para> </para>
<para> <para>
Consumers of <literal>pg_stat_statements</> may wish to use Consumers of <structname>pg_stat_statements</> may wish to use
<structfield>queryid</> (perhaps in combination with <structfield>queryid</> (perhaps in combination with
<structfield>dbid</> and <structfield>userid</>) as a more stable <structfield>dbid</> and <structfield>userid</>) as a more stable
and reliable identifier for each entry than its query text. and reliable identifier for each entry than its query text.
@ -252,7 +252,7 @@
post-parse-analysis tree, its value is a function of, among other post-parse-analysis tree, its value is a function of, among other
things, the internal object identifiers appearing in this representation. things, the internal object identifiers appearing in this representation.
This has some counterintuitive implications. For example, This has some counterintuitive implications. For example,
<literal>pg_stat_statements</> will consider two apparently-identical <filename>pg_stat_statements</> will consider two apparently-identical
queries to be distinct, if they reference a table that was dropped queries to be distinct, if they reference a table that was dropped
and recreated between the executions of the two queries. and recreated between the executions of the two queries.
The hashing process is also sensitive to differences in The hashing process is also sensitive to differences in
@ -272,6 +272,20 @@
not be a useful identifier for accumulating costs across a set of logical not be a useful identifier for accumulating costs across a set of logical
replicas. If in doubt, direct testing is recommended. replicas. If in doubt, direct testing is recommended.
</para> </para>
<para>
The representative query texts are kept in an external disk file, and do
not consume shared memory. Therefore, even very lengthy query texts can
be stored successfully. However, if many long query texts are
accumulated, the external file might grow unmanageably large. As a
recovery method if that happens, <filename>pg_stat_statements</> may
choose to discard the query texts, whereupon all existing entries in
the <structname>pg_stat_statements</> view will show
null <structfield>query</> fields, though the statistics associated with
each <structfield>queryid</> are preserved. If this happens, consider
reducing <varname>pg_stat_statements.max</varname> to prevent
recurrences.
</para>
</sect2> </sect2>
<sect2> <sect2>