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From: Hannu Krosing <hannuk@google.com>
Date: Thu, 4 Dec 2025 22:03:33 +0100
Message-ID: 
 <CAMT0RQRT2wjMtH0iDLkzqAh9=SSo+Oed7jDT3jvsHXOgjY91Gw@mail.gmail.com>
Subject: Re: Revisiting {CREATE INDEX, REINDEX} CONCURRENTLY improvements
To: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Cc: Antonin Houska <ah@cybertec.at>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
	Sergey Sargsyan <sergey.sargsyan.2001@gmail.com>, alvherre@kurilemu.de,
	Andres Freund <andres@anarazel.de>, Michael Paquier <michael@paquier.xyz>,
	Andrey Borodin <amborodin86@gmail.com>,
 Melanie Plageman <melanieplageman@gmail.com>
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 <https://www.postgresql.org/message-id/CAMT0RQRT2wjMtH0iDLkzqAh9%3DSSo%2BOed7jDT3jvsHXOgjY91Gw%40mail.gmail.com>
Precedence: bulk

I just sent a small patch for logical decoding to pgsql-hackers@
exposing to logical decoding old and new tuple ids and a boolean
telling if an UPDATE is HOT.

Feel free to test if this helps here as well

On Thu, Dec 4, 2025 at 8:15=E2=80=AFPM Antonin Houska <ah@cybertec.at> wrot=
e:
>
> Matthias van de Meent <boekewurm+postgres@gmail.com> wrote:
>
> > On Thu, 4 Dec 2025 at 09:34, Antonin Houska <ah@cybertec.at> wrote:
> > >
> > > ISTM that what you consider a problem is copying the table using PGPR=
OC-based
> > > snapshot and applying logically decoded commits to the result - is th=
at what
> > > you mean?
> >
> > Correct.
> >
> > > In fact, LR (and also REPACK) uses snapshots generated by the logical=
 decoding
> > > system. The information on running/committed transactions is based he=
re on
> > > replaying WAL, not on PGPROC.
> >
> > OK, that's good to know. For reference, do you know where this is
> > documented, explained, or implemented?
>
> All my knowledge of these things is from source code.
>
> > I'm asking, because the code that I could find didn't seem use any
> > special snapshot (tablesync.c uses
> > `PushActiveSnapshot(GetTransactionSnapshot())`),
>
> My understanding is that this is what happens on the subscription side. S=
ome
> lines above that however, walrcv_create_slot(..., CRS_USE_SNAPSHOT, ...) =
is
> called which in turn calls CreateReplicationSlot(..., CRS_USE_SNAPSHOT, .=
..)
> on the publication side and it sets that snapshot for the transaction on =
the
> remote (publication) side:
>
>         else if (snapshot_action =3D=3D CRS_USE_SNAPSHOT)
>         {
>                 Snapshot        snap;
>
>                 snap =3D SnapBuildInitialSnapshot(ctx->snapshot_builder);
>                 RestoreTransactionSnapshot(snap, MyProc);
>         }
>
> > and the other
> > reference to LR's snapshots (snapbuild.c, and inside
> > `GetTransactionSnapshot()`) explicitly said that its snapshots are
> > only to be used for catalog lookups, never for general-purpose
> > queries.
>
> I think the reason is that snapbuild.c only maintains snapshots for catal=
og
> scans, because in logical decoding you only need to scan catalog tables. =
This
> is especially to find out which tuple descriptor was valid when particula=
r
> data change (INSERT / UPDATE / DELETE) was WAL-logged - the output plugin
> needs the correct version of tuple descriptor to deform each tuple. Howev=
er
> there is no need to scan non-catalog tables: as long as wal_level=3Dlogic=
al, the
> WAL records contains all the information needed for logical replication
> (including key values). So snapbuild.c only keeps track of transactions t=
hat
> modify system catalog and uses this information to create the snapshots.
>
> A special case is if you pass need_full_snapshot=3Dtrue to
> CreateInitDecodingContext(). In this case the snapshot builder tracks com=
mits
> of all transactions, but only does so until SNAPBUILD_CONSISTENT state is
> reached. Thus, just before the actual decoding starts, you can get a snap=
shot
> to scan even non-catalog tables (SnapBuildInitialSnapshot() creates that,=
 like
> in the code above). (For REPACK, I'm trying to teach snapbuild.c recogniz=
e
> that transaction changed one particular non-catalog table, so it can buil=
d
> snapshots to scan this one table anytime.)
>
> Another reason not to use those snapshots for non-catalog tables is that
> snapbuild.c creates snapshots of the kind SNAPSHOT_HISTORIC_MVCC. If you =
used
> this for non-catalog tables, HeapTupleSatisfiesHistoricMVCC() would be us=
ed
> for visibility checks instead of HeapTupleSatisfiesMVCC(). The latter can
> handle tuples surviving from older version of postgres, but the earlier
> cannot:
>
>         /* Used by pre-9.0 binary upgrades */
>         if (tuple->t_infomask & HEAP_MOVED_OFF)
>
> No such tuples should appear in the catalog because initdb always creates=
 it
> from scratch.
>
> For LR, SnapBuildInitialSnapshot() takes care of the conversion from
> SNAPSHOT_HISTORIC_MVCC to SNAPSHOT_MVCC.
>
> --
> Antonin Houska
> Web: https://www.cybertec-postgresql.com
>
>