Re: [HACKERS] Buglist

Tom Lane wrote:

> Jan Wieck <JanWieck(at)Yahoo(dot)com> writes:
>> Shridhar Daithankar wrote:
>>> Umm.. What does FSM does then? I was under impression that FSM stores page
>>> pointers and vacuum work on FSM information only. In that case, it wouldn't
>>> have to waste time to find out which pages to clean.
>
>> It's the other way around! VACUUM scan's the tables to find and reclaim
>> free space and remembers that free space in the FSM.
>
> Right. One big question mark in my mind about these "partial vacuum"
> proposals is whether they'd still allow adequate FSM information to be
> maintained. If VACUUM isn't looking at most of the pages, there's no
> very good way to acquire info about where there's free space.

That's why I think it needs one more pg_stat column to count the number
of vacuumed tuples. If one does

tuples_updated + tuples_deleted - tuples_vacuumed

he'll get approximately the number of tuples a regular vacuum might be
able to reclaim. If that number is really small, no need for autovacuum
to cause any big trouble by scanning the relation.

Another way to give autovacuum some hints would be to return some number
as commandtuples from vacuum. like the number of tuples actually
vacuumed. That together with the new number of reltuples in pg_class
will tell autovacuum how frequent a relation really needs scanning.

Jan

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