Re: COUNT(*) and index-only scans

On Wed, Oct 12, 2011 at 2:50 AM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> On Tue, 2011-10-11 at 13:22 -0400, Robert Haas wrote:
>> The real issue is that the costing estimates need to be accurate, and
>> that's where the rubber hits the road.  Otherwise, even if we pick the
>> right way to scan the table, we may do silly things up the line when
>> we go to start constructing the join order.  I think we need to beef
>> up ANALYZE to gather statistics on the fraction of the pages that are
>> marked all-visible, or maybe VACUUM should gather that information.
>> The trouble is that if we VACUUM and then ANALYZE, we'll often get
>> back a value very close to 100%, but then the real value may diminish
>> quite a bit before the next auto-analyze fires.  I think if we can
>> figure out what to do about that problem we'll be well on our way...
>
> Can you send stats messages to keep track when you unset a bit in the
> VM? That might allow it to be more up-to-date.

In theory, that seems like it would work, although I'm a little
worried about the overhead.

--
Robert Haas
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