Re: why there is not VACUUM FULL CONCURRENTLY?

Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:

> On 2025-Apr-01, Antonin Houska wrote:
>
> > Besides that, it occurred to me that 0005 ("Preserve visibility
> > information of the concurrent data changes.") will probably introduce
> > significant overhead. The problem is that the table we're repacking is
> > treated like a catalog, for reorderbuffer.c to generate snapshots that
> > we need to replay UPDATE / DELETE commands on the new table.
> >
> > contrib/test_decoding can be used to demonstrate the difference
> > between ordinary and catalog tables:
> >
> > [.. ordinary ..]
> > Execution Time: 3521.190 ms
> > [.. catalog ..]
> > Execution Time: 6561.634 ms
>
> Significant indeed. Thinking about the scenarios in which I envision
> people using REPACK CONCURRENTLY (mostly, cases where very large tables
> have accumulated considerable amounts of bloat) and considering the size
> of the patch, I think the case for treating it as concurrent-safe is not
> credible, at least not at this stage -- not only because of this
> performance impact, but also because of the additional code complexity,
> which I'm really doubtful we can address at this stage. I would suggest
> to put that patch aside for now, maybe with a doc warning that
> "repacking a table would cause visibility information to be lost"; and
> then address that aspect later on, after this feature has gone through
> some battle-hardening.

ok, I'll adjust the patch set.

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