MIME-Version: 1.0
References: 
 <CALDaNm3tiKhtegx5Cawi34UjbHmNGEDNAtScGM1RgWRtV-5_0Q@mail.gmail.com>
 <202604060918.qw5ms7cbr2hz@alvherre.pgsql>
In-Reply-To: <202604060918.qw5ms7cbr2hz@alvherre.pgsql>
From: Amit Kapila <amit.kapila16@gmail.com>
Date: Mon, 6 Apr 2026 17:26:33 +0530
Message-ID: 
 <CAA4eK1+6GyTWSQ4XC8B=TJCkk=uFG1N+d7xg+Hp1N=uphVX0=Q@mail.gmail.com>
Subject: Re: Adding REPACK [concurrently]
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: vignesh C <vignesh21@gmail.com>, Antonin Houska <ah@cybertec.at>,
	Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
 Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Pg Hackers <pgsql-hackers@lists.postgresql.org>,
 Robert Treat <rob@xzilla.net>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Archived-At: 
 <https://www.postgresql.org/message-id/CAA4eK1%2B6GyTWSQ4XC8B%3DTJCkk%3DuFG1N%2Bd7xg%2BHp1N%3DuphVX0%3DQ%40mail.gmail.com>
Precedence: bulk

On Mon, Apr 6, 2026 at 3:08=E2=80=AFPM Alvaro Herrera <alvherre@alvh.no-ip.=
org> wrote:
>
> On 2026-Apr-06, vignesh C wrote:
>
> > On Mon, 6 Apr 2026 at 02:12, Alvaro Herrera <alvherre@alvh.no-ip.org> w=
rote:
>
> > Few comments:
>
> Thanks for reviewing this patch!
>
> > 1) Can we add a comment why we should error out here, as repack
> > concurrently requires this whereas repack does not require this check.
> > Even if it is required for decoding can't it be handled by replica
> > identity full:
> > +       /*
> > +        * If the identity index is not set due to replica identity bei=
ng, PK
> > +        * might exist.
> > +        */
> > +       ident_idx =3D RelationGetReplicaIndex(rel);
> > +       if (!OidIsValid(ident_idx) && OidIsValid(rel->rd_pkindex))
> > +               ident_idx =3D rel->rd_pkindex;
> > +       if (!OidIsValid(ident_idx))
> > +               ereport(ERROR,
> > +
> > errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> > +                               errmsg("cannot process relation \"%s\""=
,
> > +                                          RelationGetRelationName(rel)=
),
> > +                               errhint("Relation \"%s\" has no
> > identity index.",
> > +                                               RelationGetRelationName=
(rel)));
>
> Ah, I was just rewriting that comment moments ago.  I think we could
> make it work with replica identity full in theory, but I'm guessing it
> would be useless.  You really need an index that lets you locate the
> affected tuples; otherwise the replay would take forever.  If the table
> is big, that would make the whole thing unworkable.  If the table isn't
> big, then you can probably just use straight repack without too much
> disruption.
>
>         /*
>          * Obtain the replica identity index -- either one that has been =
set
>          * explicitly, or the primary key.  If none of these cases apply,=
 the
>          * table cannot be repacked concurrently.  It might be possible t=
o have
>          * repack work with a FULL replica identity; however that require=
s more
>          * work and is not implemented yet.
>          */
>
> Now, it's possible to have a non-unique index on some columns (not good
> enough to be replica identity) which gives you a list of candidate
> tuples, and then the implementation chooses one based on the other
> columns which are present in the FULL replica identity.  (This seems a
> bit dangerous, because there might be multiple matching tuples; and how
> do you choose?)  Now let's further suppose you can narrow down to a
> single tuple; in that case, using FULL would work.
>

I think this is already working for apply worker where we compare
tuples if the index is non_uniuqe, see FindReplTupleInLocalRel.

>
  However, as I said,
> this requires more implementation effort.  We could entertain a patch
> for this during the pg20 cycle, though I'm doubtful that it would really
> be worth your while.
>

It is fine to support it later in pg20. In general, I wonder if we
have evaluated whether some of the apply-worker infrastructure could
have been reused here?

--=20
With Regards,
Amit Kapila.