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doc: Mention de-normalization of deallocated entries in pg_stat_statements

The current implementation of query normalization in pg_stat_statementsis optimistic. If an entry is deallocated between the post-analyze hookand the planner and/or execution hook, it can be possible to find querystrings with literal constant values (like "SELECT 1, 2") rather thantheir normalized flavor (like "SELECT $1, $2").This commit adds in the documentation a paragraph about this limitation,and that this risk can be reduced by increasing pg_stat_statements.max,particularly if pg_stat_statements_info reports a high number ofdeallocations.Author: Sami ImseihDiscussion:https://postgr.es/m/9CFF3512-355B-4676-8CCC-6CF622F4DC1A@amazon.com

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- pgstatstatements.sgml

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`‎doc/src/sgml/pgstatstatements.sgml`

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`@@ -516,6 +516,16 @@`
`516`	`516`	`<structname>pg_stat_statements</structname> entry.`
`517`	`517`	`</para>`
`518`	`518`
	`519`	`+ <para>`
	`520`	`+ Queries on which normalization can be applied may be observed with constant`
	`521`	`+ values in <structname>pg_stat_statements</structname>, especially when there`
	`522`	`+ is a high rate of entry deallocations. To reduce the likelihood of this`
	`523`	`+ happening, consider increasing <varname>pg_stat_statements.max</varname>.`
	`524`	`+ The <structname>pg_stat_statements_info</structname> view, discussed below`
	`525`	`+ in <xref linkend="pgstatstatements-pg-stat-statements-info"/>,`
	`526`	`+ provides statistics about entry deallocations.`
	`527`	`+ </para>`
	`528`	`+`
`519`	`529`	`<para>`
`520`	`530`	`In some cases, queries with visibly different texts might get merged into a`
`521`	`531`	`single <structname>pg_stat_statements</structname> entry. Normally this will happen`

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`‎doc/src/sgml/pgstatstatements.sgml`

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