diff options
author | Tom Lane | 2015-10-05 16:44:12 +0000 |
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committer | Tom Lane | 2015-10-05 16:44:12 +0000 |
commit | f8a5e579d18b268f287d81b8a4bd54955ca90c37 (patch) | |
tree | 7d07fbecf88fbcf56b23fcc0cefc1647cf14c4a0 | |
parent | 9e36c91b468d7d821b77214337ff891811b4b337 (diff) |
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.
-rw-r--r-- | doc/src/sgml/pgstatstatements.sgml | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/doc/src/sgml/pgstatstatements.sgml b/doc/src/sgml/pgstatstatements.sgml index 4d7a6e68ea..d4f09fc2a3 100644 --- a/doc/src/sgml/pgstatstatements.sgml +++ b/doc/src/sgml/pgstatstatements.sgml @@ -270,7 +270,7 @@ </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>dbid</> and <structfield>userid</>) as a more stable and reliable identifier for each entry than its query text. @@ -280,7 +280,7 @@ post-parse-analysis tree, its value is a function of, among other things, the internal object identifiers appearing in this representation. 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 and recreated between the executions of the two queries. The hashing process is also sensitive to differences in @@ -300,6 +300,20 @@ not be a useful identifier for accumulating costs across a set of logical replicas. If in doubt, direct testing is recommended. </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> |