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Re: Adding REPACK [concurrently]

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Antonin Houska <ah@cybertec.at>
Cc: Mihail Nikalayeu <mihailnikalayeu@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>, Robert Treat <rob@xzilla.net>
Date: 2026-02-24T18:29:17Z
Lists: pgsql-hackers 
On 2026-Feb-23, Alvaro Herrera wrote:

Looking at this function in pgoutput_repack.c:

> +/* Store concurrent data change. */
> +static void
> +store_change(LogicalDecodingContext *ctx, ConcurrentChangeKind kind,
> +			 HeapTuple tuple)
> +{

[...] we have this:

> +	size = VARHDRSZ + SizeOfConcurrentChange;
> +
> +	/*
> +	 * ReorderBufferCommit() stores the TOAST chunks in its private memory
> +	 * context and frees them after having called apply_change().  Therefore
> +	 * we need flat copy (including TOAST) that we eventually copy into the
> +	 * memory context which is available to decode_concurrent_changes().
> +	 */
> +	if (HeapTupleHasExternal(tuple))
> +	{
> +		/*
> +		 * toast_flatten_tuple_to_datum() might be more convenient but we
> +		 * don't want the decompression it does.
> +		 */
> +		tuple = toast_flatten_tuple(tuple, dstate->tupdesc);
> +		flattened = true;
> +	}
> +
> +	size += tuple->t_len;
> +	if (size >= MaxAllocSize)
> +		elog(ERROR, "Change is too big.");
> +
> +	/* Construct the change. */
> +	change_raw = (char *) palloc0(size);
> +	SET_VARSIZE(change_raw, size);

I wonder if this isn't problematic with large tuples.  If a row has some
very wide columns, each of which individually is less than 1 GB, then it
might happen that the sum of their sizes exceeds 1 GB, causing palloc()
to complain and abort the whole repack operation.  This wouldn't be very
nice, so I think we need to address it somehow.

Another thing I'm not very keen on, is the fact that we have to memcpy()
the tuple contents a few lines below:

> +	/*
> +	 * Copy the tuple.
> +	 *
> +	 * Note: change->tup_data.t_data must be fixed on retrieval!
> +	 */
> +	memcpy(&change.tup_data, tuple, sizeof(HeapTupleData));
> +	memcpy(dst, &change, SizeOfConcurrentChange);
> +	dst += SizeOfConcurrentChange;
> +	memcpy(dst, tuple->t_data, tuple->t_len);

> +	/* Store as tuple of 1 bytea column. */
> +	values[0] = PointerGetDatum(change_raw);
> +	isnull[0] = false;
> +	tuplestore_putvalues(dstate->tstore, dstate->tupdesc_change,
> +						 values, isnull);

To make matters worse, tuplestore_putvalues does a
heap_form_minimal_tuple() on this and copies the data again.  This seems
pretty wasteful.

I think we need some new APIs to avoid all this copying.  It appears
that it all starts with reorderbuffer doing something unhelpful with the
memory context of the TOAST chunks.  Maybe we should address this by
"fixing" reorderbuffer so that it doesn't do this, instead of playing so
many games to cope.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Allow old WAL recycling during REPACK CONCURRENTLY

  2. Advance restart_lsn more eagerly in LogicalConfirmReceivedLocation

  3. Remove unnecessary signal handler change

  4. Improve REPACK (CONCURRENTLY) error messages some more

  5. Revert "Allow logical replication snapshots to be database-specific"

  6. Move REPACK (CONCURRENTLY) test out of stock regression tests

  7. REPACK: do not require REPLICATION or LOGIN

  8. Add missing initialization

  9. Simplify declaration of memcpy target

  10. Reserve replication slots specifically for REPACK

  11. doc: Add an example of REPACK (CONCURRENTLY)

  12. Allow logical replication snapshots to be database-specific

  13. Avoid different-size pointer-to-integer cast

  14. Fix valgrind failure

  15. Add CONCURRENTLY option to REPACK

  16. Rename cluster.c to repack.c (and corresponding .h)

  17. Allow index_create to suppress index_build progress reporting

  18. Make index_concurrently_create_copy more general

  19. Document the 'command' column of pg_stat_progress_repack

  20. Introduce the REPACK command

  21. Toggle logical decoding dynamically based on logical slot presence.

  22. Split vacuumdb to create vacuuming.c/h

  23. Remove ReorderBufferTupleBuf structure.

  24. Revert changes to CONCURRENTLY that "sped up" Xmin advance

  25. VACUUM: ignore indexing operations with CONCURRENTLY