Re: [HACKERS] Partition-wise aggregation/grouping

On Fri, Apr 28, 2017 at 3:03 AM, Antonin Houska <ah@cybertec.at> wrote:
> I think this is not generic enough because the result of the Append plan can
> be joined to another relation. As such a join can duplicate the
> already-aggregated values, the aggregates should not be finalized below the
> top-level plan.

If the grouping key matches the partition key, then it's correct to
push the entire aggregate down, and there's probably a large
performance advantage from avoiding aggregating twice.  If the two
don't match, then pushing the aggregate down necessarily involves a
"partial" and a "finalize" stage, which may or may not be cheaper than
doing the aggregation all at once.  If you have lots of 2-row groups
with 1 row in the first branch of the append and 1 row in the second
branch of the append, breaking the aggregate into two steps is
probably going to be a loser.  If the overall number of groups is
small, it's probably going to win.  But when the grouping key matches
the partition key, so that two-stage aggregation isn't required, I
suspect the pushdown should almost always win.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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