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docs: mention that expression indexes need analyze

Expression indexes can't benefit from pre-computed statistics oncolumns.Reported-by: Nikolay SamokhvalovDiscussion:https://postgr.es/m/CANNMO++5rw9RDA=p40iMVbMNPaW6O=S0AFzTU=KpYHRpCd1voA@mail.gmail.comAuthor: Nikolay Samokhvalov, modifiedBackpatch-through: 9.5

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`@@ -741,6 +741,16 @@ Indexes:`
`741`	`741`	`sort high</quote>, in queries that depend on indexes to avoid sorting steps.`
`742`	`742`	`</para>`
`743`	`743`
	`744`	`+ <para>`
	`745`	`+ The regularly system collects statistics on all of a table's`
	`746`	`+ columns. Newly-created non-expression indexes can immediately`
	`747`	`+ use these statistics to determine an index's usefulness.`
	`748`	`+ For new expression indexes, it is necessary to run <link`
	`749`	`+ linkend="sql-analyze"><command>ANALYZE</command></link> or wait for`
	`750`	`+ the <link linkend="autovacuum">autovacuum daemon</link> to analyze`
	`751`	`+ the table to generate statistics about new expression indexes.`
	`752`	`+ </para>`
	`753`	`+`
`744`	`754`	`<para>`
`745`	`755`	`For most index methods, the speed of creating an index is`
`746`	`756`	`dependent on the setting of <xref linkend="guc-maintenance-work-mem"/>.`

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