Re: "Compacting" a relation

On Sat, 2007-02-03 at 22:11 -0500, Bruce Momjian wrote:
> Tom Lane wrote:
> > Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> > > vacuumlazy.c contains a hint "Consider compacting this relation" but AFAICT,
> > > there is no indication anywhere how "compacting" is supposed to be achieved.
> > > I guess this means VACUUM FULL or CLUSTER, but I don't think the hint can be
> > > processed effectively by a user.
> >
> > So change it ...
>
> New message is:
>
> errhint("Consider using VACUUM FULL on this relation or increasing the configuration parameter \"max_fsm_pages\".")));
>

The change of wording may be appropriate, but it is triggered when

if (vacrelstats->tot_free_pages > MaxFSMPages)

So if you VACUUM a 15+GB table and it has only 1% freespace then it will
still generate this message. Hopefully you'd agree that the message
would be inappropriate in that case.

It's also inappropriate because this message is generated *prior* to
doing lazy_truncate_heap(), which could easily remove lots of empty
pages anyhow. That might reduce it to less than MaxFSMPages anyhow, so
it can currently be triggered in wholly inappropriate situations.

So I suggest that we move this wording after lazy_truncate_heap() in
lazy_vacuum_rel() *and* we alter the hint so that it only suggests
VACUUM FULL if the table has 20% fragmentation, whatever its size.

Happy to drop a patch for this, if people agree.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com