Re: Improve hash-agg performance

On 2016-11-04 15:18:49 +0200, Heikki Linnakangas wrote:
> On 11/03/2016 01:07 PM, Andres Freund wrote:
> > Hi,
> >
> > There's two things I found while working on faster expression
> > evaluation, slot deforming and batched execution. As those two issues
> > often seemed quite dominant cost-wise it seemed worthwhile to evaluate
> > them independently.
> >
> > 1) We atm do one ExecProject() to compute each aggregate's
> > arguments. Turns out it's noticeably faster to compute the argument
> > for all aggregates in one go. Both because it reduces the amount of
> > function call / moves more things into a relatively tight loop, and
> > because it allows to deform all the required columns at once, rather
> > than one-by-one. For a single aggregate it'd be faster to avoid
> > ExecProject alltogether (i.e. directly evaluate the expression as we
> > used to), but as soon you have two the new approach is faster.
>
> Makes sense. If we do your refactoring of ExecEvalExpr into an intermediate
> opcode representation, I assume the performance difference will go away
> anyway.

Precisely.

> > 2) For hash-aggs we right now we store the representative tuple using
> > the input tuple's format, with unneeded columns set to NULL. That
> > turns out to be expensive if the aggregated-on columns are not
> > leading columns, because we have to skip over a potentially large
> > number of NULLs. The fix here is to simply use a different tuple
> > format for the hashtable. That doesn't cause overhead, because we
> > already move columns in/out of the hashslot explicitly anyway.

> Heh, I came to the same conclusion a couple of months ago when I was
> profiling the aggregate code. I never got around to finish up and post the
> patch I wrote back then, but here you go, for comparison. It's pretty much
> the same as what you got here. So yeah, seems like a good idea.

> + /*
> + * Note that we don't deduplicate key columns. That would complicate
> + * the comparisons. Don't write silly queries! (Not sure if that would get
> + * through the parser and optimizer, though).

Hehe. You made me spill more coffee.

Thanks for looking!

Greetings,

Andres Freund