From: Antonin Houska <ah@cybertec.at>
To: Andres Freund <andres@anarazel.de>
cc: pgsql-hackers@lists.postgresql.org,
    Alvaro Herrera <alvherre@2ndquadrant.com>,
    pgsql-hackers@postgresql.org
Subject: Re: Incorrect visibility test function assigned to snapshot
In-reply-to: <B6BD7989-0298-4E05-B5AA-9B97843826BE@anarazel.de>
References: <20180530132854.tkv37qiamepu3slh@alvherre.pgsql>
 <28218.1527687932@localhost>
 <B6BD7989-0298-4E05-B5AA-9B97843826BE@anarazel.de>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
   message dated "Wed, 30 May 2018 09:49:44 -0400."
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Date: Wed, 30 May 2018 16:00:44 +0200
Message-ID: <28524.1527688844@localhost>
Precedence: bulk

Andres Freund <andres@anarazel.de> wrote:

> On May 30, 2018 9:45:32 AM EDT, Antonin Houska <ah@cybertec.at> wrote:
> >Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
> >
> >> On 2018-May-30, Antonin Houska wrote:
> >>=20
> >> > In the header comment, SnapBuildInitialSnapshot() claims to set
> >> > snapshot->satisfies to the HeapTupleSatisfiesMVCC test function,
> >and indeed it
> >> > converts the "xid" array to match its semantics (i.e. the xid items
> >eventually
> >> > represent running transactions as opposed to the committed ones).
> >However the
> >> > test function remains HeapTupleSatisfiesHistoricMVCC as set by
> >> > SnapBuildBuildSnapshot().
> >>=20
> >> Interesting.  While this sounds like an oversight that should have
> >> horrible consequences, it's seems not to because the current callers
> >> don't seem to care about the ->satisfies function.  Are you able to
> >come
> >> up with some scenario in which it causes an actual problem?
> >
> >Right, the current callers in the core do not seem to use that
> >function. I hit
> >the issue when doing and testing some changes in an extension
> >(pg_squeeze).
>=20
> What is that extension doing with that snapshot?

It fetches data from a table in order to insert them into a new table, to
eliminate bloat. Something like pg_repack, but it uses logical decoding
instead of triggers to capture concurrent data changes.

--=20
Antonin Houska
Cybertec Sch=C3=B6nig & Sch=C3=B6nig GmbH
Gr=C3=B6hrm=C3=BChlgasse 26, A-2700 Wiener Neustadt
Web: https://www.cybertec-postgresql.com