Re: Autovacuum loose ends

Tom Lane <tgl@sss.pgh.pa.us> writes:

> I was thinking GUC settings only; is there a real use-case for
> table-specific delay parameters?  ISTM the point of the delay parameters
> for autovac is to put a lid on its impact on interactive response.  Seen
> in that light, you do not care exactly which table it's hitting at the
> moment.

I'm not sure that's true.

ISTM if you have a small table that needs to be vacuumed frequently you
probably don't want it taking longer than necessary to vacuum. It's probably
mostly cached so there wouldn't be much of an i/o hit and even a small sleep
can make a big proportional difference in vacuum run time. You could get into
a situation where it takes longer to vacuum a bunch of such tables than the
frequency you need the vacuuming to taking place.

I think the i/o problem comes when you have large uncached tables. They
probably have a relatively small percentage of the table being updated and so
don't need to be vacuumed frequently. But when they do you need the sleeps to
avoid the i/o problems.

-- 
greg