Thread

Re: Revisiting {CREATE INDEX, REINDEX} CONCURRENTLY improvements

Hannu Krosing <hannuk@google.com> — 2025-12-04T21:03:33Z

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 PM Antonin Houska <ah@cybertec.at> wrote:
>
> 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 PGPROC-based
> > > snapshot and applying logically decoded commits to the result - is that 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 here 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. Some
> 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 == CRS_USE_SNAPSHOT)
>         {
>                 Snapshot        snap;
>
>                 snap = 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 catalog
> scans, because in logical decoding you only need to scan catalog tables. This
> is especially to find out which tuple descriptor was valid when particular
> data change (INSERT / UPDATE / DELETE) was WAL-logged - the output plugin
> needs the correct version of tuple descriptor to deform each tuple. However
> there is no need to scan non-catalog tables: as long as wal_level=logical, the
> WAL records contains all the information needed for logical replication
> (including key values). So snapbuild.c only keeps track of transactions that
> modify system catalog and uses this information to create the snapshots.
>
> A special case is if you pass need_full_snapshot=true to
> CreateInitDecodingContext(). In this case the snapshot builder tracks commits
> of all transactions, but only does so until SNAPBUILD_CONSISTENT state is
> reached. Thus, just before the actual decoding starts, you can get a snapshot
> to scan even non-catalog tables (SnapBuildInitialSnapshot() creates that, like
> in the code above). (For REPACK, I'm trying to teach snapbuild.c recognize
> that transaction changed one particular non-catalog table, so it can build
> 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 used
> 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
>
>