Tom Lane wrote:
> Greg Stark <gsstark@mit.edu> writes:
>
>>Last I heard the reason count(*) was so expensive was because its state
>>variable was a bigint. That means it doesn't fit in a Datum and has to be
>>alloced and stored as a pointer. And because of the Aggregate API that means
>>it has to be allocated and freed for every tuple processed.
>
>
> There's a hack in 8.1 to avoid the palloc overhead (courtesy of Neil
> Conway IIRC).
>
It certainly makes quite a difference as I measure it:
doing select(1) from a 181000 page table (completely uncached) on my PIII:
8.0 : 32 s
8.1 : 25 s
Note that the 'fastcount()' function takes 21 s in both cases - so all
the improvement seems to be from the count overhead reduction.
Cheers
Mark
