Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Mar 15, 2010 at 3:04 PM, Gregg Lind <gregg.lind@gmail.com> wrote:
>> As per http://osdir.com/ml/pgsql-hackers/2010-03/msg00331.html, I would
>> certainly find it helpful is count(distinct(myfield)) wasn't a full index
>> scan.
> We should maybe add this to our Todo file, if it's not there already
> (I don't see it on a quick look).
I think previous discussions of it have been under names like "skip
scan" (I certainly find "loose scan" to be completely useless
terminology, if indeed it even means what the OP wants at all).
However, most of the complaints about this were logged against PG
versions that wouldn't convert DISTINCT queries into hash aggregation.
COUNT(DISTINCT x) still doesn't. Sometime it might be worth trying
to fix that.
regards, tom lane