From e96b697adb138b0bbe34082761b4e4538abcc9e6 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Mon, 5 Oct 2015 12:44:12 -0400 Subject: [PATCH] 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. --- doc/src/sgml/pgstatstatements.sgml | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/doc/src/sgml/pgstatstatements.sgml b/doc/src/sgml/pgstatstatements.sgml index a58ecf7065f..ca9478acdf6 100644 --- a/doc/src/sgml/pgstatstatements.sgml +++ b/doc/src/sgml/pgstatstatements.sgml @@ -242,7 +242,7 @@ - Consumers of pg_stat_statements may wish to use + Consumers of pg_stat_statements may wish to use queryid (perhaps in combination with dbid and userid) as a more stable 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 things, the internal object identifiers appearing in this representation. This has some counterintuitive implications. For example, - pg_stat_statements will consider two apparently-identical + 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 @@ -272,6 +272,20 @@ not be a useful identifier for accumulating costs across a set of logical replicas. If in doubt, direct testing is recommended. + + + 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, pg_stat_statements may + choose to discard the query texts, whereupon all existing entries in + the pg_stat_statements view will show + null query fields, though the statistics associated with + each queryid are preserved. If this happens, consider + reducing pg_stat_statements.max to prevent + recurrences. +