Mark Kirkwood <markir@paradise.net.nz> writes:
> Yeah - it's pretty clear that the count aggregate is fairly expensive wrt cpu -
> However, I am not sure if all agg nodes suffer this way (guess we could try a
> trivial aggregate that does nothing for all tuples bar the last and just
> reports the final value it sees).
As you mention count(*) and count(1) are the same thing.
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 was some talk of having a special case API for count(*) and maybe
sum(...) to avoid having to do this.
There was also some talk of making Datum 8 bytes wide on platforms where that
was natural (I guess AMD64, Sparc64, Alpha, Itanic).
Afaik none of these items have happened but I don't know for sure.
--
greg
