Re: Improving avg performance for numeric

I wrote:
> [ looks at patch... ] Oh, I see what's affecting the plan: you changed
> the aggtranstypes to internal for a bunch of aggregates. That's not
> very good, because right now the planner takes that to mean that the
> aggregate could eat a lot of space. We don't want that to happen for
> these aggregates, I think.

After thinking about that for awhile: if we pursue this type of
optimization, what would probably be appropriate is to add an aggregate
property (stored in pg_aggregate) that allows direct specification of
the size that the planner should assume for the aggregate's transition
value. We were getting away with a hardwired assumption of 8K for
"internal" because the existing aggregates that used that transtype all
had similar properties, but it was always really a band-aid not a proper
solution. A per-aggregate override could be useful in other cases too.

This was looking like 9.4 material already, but adding such a property
would definitely put it over the top of what we could think about
squeezing into 9.3, IMO.

regards, tom lane