Re: Adding REPACK [concurrently]

Antonin Houska <ah@cybertec.at>

From: Antonin Houska <ah@cybertec.at>

To: Mihail Nikalayeu <mihailnikalayeu@gmail.com>

Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Fujii Masao <masao.fujii@gmail.com>, Robert Treat <rob@xzilla.net>, Pg Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2025-08-21T18:07:04Z

Lists: pgsql-hackers

Commits

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

Improve REPACK (CONCURRENTLY) error messages some more
- 378dffaf8c80 19 (unreleased) landed
Add CONCURRENTLY option to REPACK
- 28d534e2ae0a 19 (unreleased) landed
Make index_concurrently_create_copy more general
- 33bf7318f94c 19 (unreleased) landed
Document the 'command' column of pg_stat_progress_repack
- a630ac5c2016 19 (unreleased) landed
Introduce the REPACK command
- ac58465e0618 19 (unreleased) landed
Split vacuumdb to create vacuuming.c/h
- c4067383cb2c 19 (unreleased) landed
Revert changes to CONCURRENTLY that "sped up" Xmin advance
- 042b584c7f7d 14.4 cited

Mihail Nikalayeu <mihailnikalayeu@gmail.com> wrote:

> Also, I think I found an issue (or lost something during rebase): we
> must preserve xmin,cmin during initial copy
> to make sure that data is going to be visible by snapshots of
> concurrent changes later:
> 
> static void
> reform_and_rewrite_tuple(......)
> .....
>       /*It is also crucial to stamp the new record with the exact same
> xid and cid,
>       * because the tuple must be visible to the snapshot of the
> applied concurrent
>       * change later.
>       */
>       CommandId      cid = HeapTupleHeaderGetRawCommandId(tuple->t_data);
>       TransactionId   xid = HeapTupleHeaderGetXmin(tuple->t_data);
> 
>       heap_insert(NewHeap, copiedTuple, xid, cid, HEAP_INSERT_NO_LOGICAL, NULL);

When posting version 12 of the patch [1] I raised a concern that the the MVCC
safety is too expensive when it comes to logical decoding. Therefore, I
abandoned the concept for now, and v13 [2] uses plain heap_insert(). Once we
implement the MVCC safety, we simply rewrite the tuple like v12 did - that's
the simplest way to preserve fields like xmin, cmin, ...

[1] https://www.postgresql.org/message-id/178741.1743514291%40localhost
[2] https://www.postgresql.org/message-id/97795.1744363522%40localhost

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com