References: <87tvh9xa0g.fsf@ars-thinkpad>
 <20190212220022.GA16807@alvherre.pgsql> <87r2c8xocq.fsf@ars-thinkpad>
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From: Arseny Sher <a.sher@postgrespro.ru>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>,
 Alexander Lakhin <a.lakhin@postgrespro.ru>
Subject: Re: Too rigorous assert in reorderbuffer.c
In-reply-to: <87r2c8xocq.fsf@ars-thinkpad>
Date: Thu, 19 Dec 2019 19:30:10 +0300
Message-ID: <87lfr8wa5p.fsf@ars-thinkpad>
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Arseny Sher <a.sher@postgrespro.ru> writes:

> I'm sorry to bother you with this again, but due to new test our
> internal buildfarm revealed that ajacent assert on cmin is also lie. You
> see, we can't assume cmin is stable because the same key (relnode, tid)
> might refer to completely different tuples if tuple was inserted by
> aborted subxact, immeditaly reclaimed and then space occupied by another
> one. Fix is attached.
>
> Technically this might mean a user-facing bug, because we only pick the
> first cmin which means we might get visibility wrong, allowing to see
> some version too early (i.e real cmin of tuple is y, but decoding thinks
> it is x, and x < y). However, I couldn't quickly make up an example
> where this would actually lead to bad consequences. I tried to create
> such extra visible row in pg_attribute, but that's ok because loop in
> RelationBuildTupleDesc spins exactly natts times and ignores what is
> left unscanned. It is also ok with pg_class, because apparently
> ScanPgRelation also fishes out the (right) first tuple and doesn't check
> for duplicates appearing later in the scan. Maybe I just haven't tried
> hard enough though.

This issue still exists, it would be nice to fix it...

--
Arseny Sher
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company