Re: Autovacuum in the backend

Hannu Krosing wrote:
> On K, 2005-06-15 at 13:41 +0800, Qingqing Zhou wrote:
> > "Neil Conway" <neilc(at)samurai(dot)com> writes
> > >
> > > Wasn't the plan to rewrite pg_autovacuum to use the FSM rather than the
> > > stats collector?
> > >
> >
> > I don't understand. Currently the basic logic of pg_autovacuum is to use the
> > pg_stat_all_tables numbers like n_tup_upd, n_tup_del to determine if a
> > relation need to be vacuumed. How to use FSM to get these information?
>
> One can't probably use FSM as it is, as FSM is filled in by vacuum and
> this creates a circular dependency.
>
> But it would be very nice to have something _similar_ to FSM, say DSM
> (dead space map), which is filled in when a tuple is marked as "dead for
> all running backends", which could be used to implement a vacuum which
> vacuums only those pages, which do actually contain removable tuples.

Yes, those are step five. The TODO list has:

* Auto-vacuum
o Move into the backend code
o Scan the buffer cache to find free space or use background writer
o Use free-space map information to guide refilling
o Do VACUUM FULL if table is nearly empty?

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