Re: possible optimization: push down aggregates

Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
> associative bit just makes it easier (which is important of course!).
> mean for example can be pushed down if the 'pushed down' aggregates
> return to the count to the "reaggregator" so that you can weight the
> final average. that's a lot more complicated though.

The real question is what you're expecting to get out of such an
"optimization". If the aggregate has to visit all rows then it's
not apparent to me that any win emerges from the extra complication.

We do already have optimization of min/max across inheritance trees,
and that's certainly a win because you don't have to visit all rows.

regression=# create table pp(f1 int unique);
CREATE TABLE
regression=# create table cc(unique(f1)) inherits(pp);
CREATE TABLE
regression=# create table cc2(unique(f1)) inherits(pp);
CREATE TABLE
regression=# explain select max(f1) from pp;
QUERY PLAN
------------------------------------------------------------------------------------------------------------
Result (cost=0.51..0.52 rows=1 width=0)
InitPlan 1 (returns $0)
-> Limit (cost=0.46..0.51 rows=1 width=4)
-> Merge Append (cost=0.46..267.71 rows=4777 width=4)
Sort Key: pp.f1
-> Index Only Scan Backward using pp_f1_key on pp (cost=0.12..8.14 rows=1 width=4)
Index Cond: (f1 IS NOT NULL)
-> Index Only Scan Backward using cc_f1_key on cc (cost=0.15..85.94 rows=2388 width=4)
Index Cond: (f1 IS NOT NULL)
-> Index Only Scan Backward using cc2_f1_key on cc2 (cost=0.15..85.94 rows=2388 width=4)
Index Cond: (f1 IS NOT NULL)
Planning time: 0.392 ms
(12 rows)

That doesn't currently extend to the GROUP BY case unfortunately.

regards, tom lane