Re: 9.5: Memory-bounded HashAgg

On Fri, 2014-08-15 at 13:53 -0400, Robert Haas wrote:
> I think that's right, and I rather like your (Jeff's) approach. It's
> definitely true that we could do better if we have a mechanism for
> serializing and deserializing group states, but (1) I think an awful
> lot of cases would get an awful lot better even just with the approach
> proposed here and (2) I doubt we would make the
> serialization/deserialization interfaces mandatory, so even if we had
> that we'd probably want a fallback strategy anyway.

Thank you for taking a look.

To solve the problem for array_agg, that would open up two potentially
lengthy discussions:

1. Trying to support non-serialized representations (like
ArrayBuildState for array_agg) as a real type rather than using
"internal".

2. What changes should we make to the aggregate API? As long as we're
changing/extending it, should we go the whole way and support partial
aggregation[1] (particularly useful for parallelism)?

Both of those discussions are worth having, and perhaps they can happen
in parallel as I wrap up this patch.

I'll see whether I can get consensus that my approach is (potentially)
commit-worthy, and your statement that it (potentially) solves a real
problem is a big help.

Regards,
Jeff Davis

[1]
http://blogs.msdn.com/b/craigfr/archive/2008/01/18/partial-aggregation.aspx