Mark Kirkwood <[EMAIL PROTECTED]> 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


---------------------------(end of broadcast)---------------------------
TIP 1: if posting/reading through Usenet, please send an appropriate
       subscribe-nomail command to [EMAIL PROTECTED] so that your
       message can get through to the mailing list cleanly

Reply via email to