Re: REPACK (CONCURRENTLY) rewrites tables marked with user_catalog_table

Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:

> Hi,
>
> On Thu, Aug 27, 2026 at 7:22 AM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
> >
> > IIUC this can cause problems for logical replication consumers, since the
> > CONCURRENTLY path doesn't go through the rewriteheap.c machinery like the
> > non-concurrent path does:
> >
> > CREATE TABLE t (a INT PRIMARY KEY, b TEXT) WITH (user_catalog_table = true);
> > ALTER TABLE t ALTER COLUMN b TYPE varchar(100); -- correctly fails
> > REPACK (CONCURRENTLY) t; -- should fail but doesn't
> >
> > I believe the fix for v19 is to add a check for
> > RelationIsUsedAsCatalogTable() in check_concurrent_repack_requirements()
> > (plus a corresponding update to the list of CONCURRENTLY limitations in the
> > docs).
>
> That's correct. I verified that concurrent repack doesn't write the
> pg_logical/mappings files, which decoding later needs to correctly map
> the old tuple locations to the new ones after a rewrite, unlike
> non-concurrent CLUSTER, VACUUM FULL and REPACK on a user catalog
> table. I think restricting user catalog tables, rather than making
> concurrent repack support them, is the right choice at this point in
> the release cycle.
>
> Please find attached a patch with the suggested doc change. I didn't
> add a test, I don't think we need one.

Thanks! I'm just thinking about this comment:

+ /* The CONCURRENTLY path does not write logical rewrite mappings. */

IMO it makes no sense to rewrite the logical mappings as long as REPACK
(CONCURRENTLY) changes visibility information (i.e. it's MVCC-unsafe). Once we
implement the MVCC-safety, we should not remove this check unless we implement
the rewriting of the mappings for user catalog tables - that might also be
worth mentioning in the comment.

So far, I'd consider the MVCC-unsafety the primary reason to prohibit REPACK
(CONCURRENTLY) (or anyother MVCC-unsafe command) on the user catalog tables,
because it can make the contents of those tables invisible to the output
plugin at some point.

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