Tom Lane wrote:
Greg Stark <gsstark@xxxxxxx> 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
