Re: hashagg slowdown due to spill changes

On Fri, 2020-06-05 at 21:11 -0700, Andres Freund wrote:
> Before there was basically one call from nodeAgg.c to execGrouping.c
> for
> each tuple and hash table. Now it's a lot more complicated:
> 1) nodeAgg.c: prepare_hash_slot()
> 2) execGrouping.c: TupleHashTableHash()
> 3) nodeAgg.c: lookup_hash_entry()
> 4) execGrouping.c: LookupTupleHashEntryHash()

The reason that I did it that way was to be able to store the hash
along with the saved tuple (similar to what HashJoin does), which
avoids recalculation.

That could be a nice savings for some cases, like when work_mem is
small but the data still fits in system memory, which I expect to be
fairly common. But based on your numbers, it might be a bad trade-off
overall.

> Why isn't the flow more like this:
> 1) prepare_hash_slot()
> 2) if (aggstate->hash_spill_mode) goto 3; else goto 4
> 3) entry = LookupTupleHashEntry(&hash); if (!entry)
> hashagg_spill_tuple();
> 4) InsertTupleHashEntry(&hash, &isnew); if (isnew) initialize(entry)

I'll work up a patch to refactor this. I'd still like to see if we can
preserve the calculate-hash-once behavior somehow.

Regards,
Jeff Davis