Re: Too rigorous assert in reorderbuffer.c

Arseny Sher <a(dot)sher(at)postgrespro(dot)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