Re: BUG #17717: Regression in vacuumdb (15 is slower than 10/11 and possible memory issue)

On Fri, Dec 16, 2022 at 6:49 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > I have to imagine that the vast majority of individual calls to
> > vac_update_datfrozenxid have just about zero chance of updating
> > datfrozenxid or datminmxid as things stand.
>
> That is a really good point.  How about teaching VACUUM to track
> the oldest original relfrozenxid and relminmxid among the table(s)
> it processed, and skip vac_update_datfrozenxid unless at least one
> of those matches the database's values?  For extra credit, also
> skip if we didn't successfully advance the source rel's value.

Hmm. I think that that would probably work.

It would certainly work on 15+, because there tends to be "natural
diversity" among the relfrozenxid values seen for each table, due to
the "track oldest extant XID" work; we no longer see many tables that
all have the same relfrozenxid, that advance in lockstep. But even
that factor probably doesn't matter, since we only need one "laggard
relfrozenxid" static table for the scheme to work and work well. That
is probably a safe bet on all versions, though I'd have to check to be
sure.

> This might lead to a fix that solves the OP's problem while not
> changing anything fundamental, which would make it reasonable
> to back-patch.

That's a big plus. This is a nasty regression. I wouldn't call it a
must-fix, but it's bad enough to be worth fixing if we can come up
with a reasonably non-invasive approach.

-- 
Peter Geoghegan