From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Paquier <michael@paquier.xyz>
cc: Postgres hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: REINDEX INDEX results in a crash for an index of pg_class since
 9.6
In-reply-to: <13930.1556063692@sss.pgh.pa.us>
References: <20190418011430.GA19133@paquier.xyz>
 <18233.1556052439@sss.pgh.pa.us> <20190423233540.GB1878@paquier.xyz>
 <13930.1556063692@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Tue, 23 Apr 2019 19:54:52 -0400"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <14270.1556064217.1@sss.pgh.pa.us>
Date: Tue, 23 Apr 2019 20:03:37 -0400
Message-ID: <14271.1556064217@sss.pgh.pa.us>
Precedence: bulk

I wrote:
> It also seems quite odd that it doesn't fail every time; surely it's
> not conditional whether we'll try to insert a new pg_class tuple or not?
> We need to understand that, too.

Oh!  One gets you ten it "works" as long as the pg_class update is a
HOT update, so that we don't actually end up touching the indexes.
This explains why the crash is less likely to happen in a database
where one's done some work (and, probably, created some dead space in
pg_class).  On the other hand, it doesn't quite fit the observation
that a VACUUM FULL masked the problem ... wouldn't that have ended up
with densely packed pg_class?  Maybe not, if it rebuilt everything
else after pg_class...

			regards, tom lane