Re: Indexes on Expressions - a note to remind users to run ANALYZE after creation

On Mon, Jul 19, 2021 at 12:59:10PM +0200, Laurenz Albe wrote:
> On Fri, 2021-07-16 at 22:00 -0700, Nikolay Samokhvalov wrote:
> > diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml
> > index 671299ff059d972ff95bdb1d67ed4c89bf5040b2..c7eaf9a608e995ef9957b4e0f677b36a8303be55 100644
> > --- a/doc/src/sgml/indices.sgml
> > +++ b/doc/src/sgml/indices.sgml
> > @@ -741,6 +741,15 @@ CREATE INDEX people_names ON people ((first_name || ' ' || last_name));
> > query. Thus, indexes on expressions are useful when retrieval speed
> > is more important than insertion and update speed.
> > </para>
> > +
> > + <note>
> > + <title>Note</title>
> > + <para>
> > + Once an index on an expression is successfully created, it is important to
> > + run <literal>ANALYZE</literal> on the corresponding table to gather
> > + statistics for the expression.
> > + </para>
> > + </note>
> > </sect1>
>
> I agree in principle. It should be "has been created" rather than "is created",
> and I would say something less drastic like "usually a good idea" rather than
> "important".

Uh, CREATE INDEX already has this mentioend:

The system regularly collects statistics on all of a table's
columns. Newly-created non-expression indexes can immediately
use these statistics to determine an index's usefulness.
For new expression indexes, it is necessary to run <link
linkend="sql-analyze"><command>ANALYZE</command></link> or wait for
the <link linkend="autovacuum">autovacuum daemon</link> to analyze
the table to generate statistics for these indexes.

Do we need it here too?

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Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

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