Date: Tue, 19 Oct 2021 12:16:44 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Michael Paquier <michael@paquier.xyz>
Cc: Euler Taveira <euler@eulerto.com>,
	"nikolai.berkoff" <nikolai.berkoff@pm.me>,
	"pgsql-docs@lists.postgresql.org" <pgsql-docs@lists.postgresql.org>
Subject: Re: ALTER TABLE ... SET DATA TYPE removes statistics
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On 2021-Oct-19, Michael Paquier wrote:

> On Mon, Oct 18, 2021 at 05:15:59PM -0300, Euler Taveira wrote:
> > I agree that it might surprise an user and it would be good to document it.
> > However, it does not belong to the description. I would add it to the Notes
> > section at the end of the ALTER TABLE page.
> 
> No objections to the suggested addition and the location of the
> addition (paragraph of SET DATA TYPE rather than "Notes"), but I think
> that the phrasing could be better:
> "The column's statistics are removed, hence a follow-up ANALYZE is
> suited to update the statistics to the new column type."

Dunno, putting it in the middle of the existing paragraph looks odd to
me.  I would put it in a separate one instead, as in the attached.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/

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diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 81291577f8..86fc15c8b3 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -198,6 +198,12 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       clause must be provided if there is no implicit or assignment
       cast from old to new type.
      </para>
+
+     <para>
+      When this form is used, the column's statistics are removed,
+      so running <command>ANALYZE</command> on the table afterwards
+      is recommended.
+     </para>
     </listitem>
    </varlistentry>
 

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