Re: COUNT(*) and index-only scans

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> Bruce Momjian <bruce@momjian.us> wrote:
>> I talked to Robert Haas and he said that index-only scans do not
>> optimize COUNT(*).  Is this something we can do for PG 9.2?  Is
>> anyone working on this?
> Well, it's not that it doesn't optimize COUNT(*) -- it's that it
> doesn't yet cost the index scan as cheaper than a table scan when
> you're accessing every row.

I think what Robert is complaining about is that we won't currently
consider an index that matches neither any WHERE clauses nor ORDER BY,
ie, count(*) over the whole table won't get considered for an index-only
scan, regardless of cost estimates.
        regards, tom lane