Date: Mon, 9 Nov 2020 18:27:20 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Nikolay Samokhvalov <samokhvalov@gmail.com>
Cc: fabriziomello@gmail.com,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Add important info about ANALYZE after create Functional Index
Message-ID: <20201109232720.GA8930@momjian.us>
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On Tue, Oct 27, 2020 at 12:12:00AM -0700, Nikolay Samokhvalov wrote:
> On Mon, Oct 26, 2020 at 3:08 PM Fabrízio de Royes Mello <
> fabriziomello@gmail.com> wrote:
> 
>     Would be nice if add some information about it into our docs but not sure
>     where. I'm thinking about:
>     - doc/src/sgml/ref/create_index.sgml
>     - doc/src/sgml/maintenance.sgml (routine-reindex)
> 
> 
> Attaching the patches for the docs, one for 11 and older, and another for 12+
> (which have REINDEX CONCURRENTLY not suffering from lack of ANALYZE).
> 
> I still think that automating is the right thing to do but of course, it's a
> much bigger topic that a quick fix dor the docs.

I see REINDEX CONCURRENTLY was fixed in head, but the docs didn't get
updated to mention the need to run ANALYZE or wait for autovacuum before
expression indexes can be fully used by the optimizer.  Instead of
putting this mention in the maintenance section, I thought the CREATE
INDEX page make more sense, since it is more of a usability issue,
rather than "why use expression indexes".  Patch attached, which I plan
to apply to all supported branches.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee


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diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml
new file mode 100644
index 749db28..48c42db
*** a/doc/src/sgml/ref/create_index.sgml
--- b/doc/src/sgml/ref/create_index.sgml
*************** Indexes:
*** 746,751 ****
--- 746,761 ----
    </para>
  
    <para>
+    The system 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 about new expression indexes.
+   </para>
+ 
+   <para>
     For most index methods, the speed of creating an index is
     dependent on the setting of <xref linkend="guc-maintenance-work-mem"/>.
     Larger values will reduce the time needed for index creation, so long

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