Message-ID: <7bbc12fe52d8907bd6c8a1421e30c6d3154ac42f.camel@cybertec.at>
Subject: Re: Use of inefficient index in the presence of dead tuples
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Alexander Staubo <alex@purefiction.net>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: "pgsql-general@postgresql.org" <pgsql-general@postgresql.org>
Date: Wed, 29 May 2024 17:10:27 +0200
In-Reply-To: <2C67231C-0A63-4B9D-AA9D-8BF69D29BC3C@purefiction.net>
References: <DC43B9C3-7BCB-4671-A69E-B0061C710241@purefiction.net>
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On Wed, 2024-05-29 at 14:36 +0200, Alexander Staubo wrote:
> > On 29 May 2024, at 02:53, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > I'm unpersuaded by the idea that ANALYZE should count dead tuples.
> > Since those are going to go away pretty soon, we would risk
> > estimating on the basis of no-longer-relevant stats and thus
> > creating problems worse than the one we solve.
>=20
> Mind you, =E2=80=9Cpretty soon=E2=80=9D could actually be =E2=80=9Chours"=
 if a pg_dump is running,
> or some other long-running transaction is holding back the xmin. Granted,
> long-running transactions should be avoided, but they happen, and the
> result is operationally surprising.

Don't do these things on a busy transactional database.

> I have another use case where I used a transaction to do lock a resource
> to prevent concurrent access. I.e. the logic did
> =E2=80=9CSELECT =E2=80=A6 FROM =E2=80=A6 WHERE id =3D $1 FOR UPDATE=E2=80=
=9D and held that transaction open
> for hours while doing maintenance.

That's a dreadful idea.

>=20
> Just to clarify, this is a real use case, though the repro is of course
> artificial since the real production case is inserting and deleting rows
> very quickly.

No doubt.
Still I think that your main trouble are long-running transactions.
They will always give you trouble on a busy PostgreSQL database.
You should avoid them.

Yours,
Laurenz Albe