2014-08-27 22:27 GMT+02:00 Tom Lane <t...@sss.pgh.pa.us>:

> Merlin Moncure <mmonc...@gmail.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.
>

I expect a remove a hashing or sorting part of aggregation. It can reduce
aggregation to seq scan only.

Pavel


>
> 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
>

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