2014-08-27 21:46 GMT+02:00 Claudio Freire <klaussfre...@gmail.com>: > On Wed, Aug 27, 2014 at 4:41 PM, Merlin Moncure <mmonc...@gmail.com> > wrote: > > On Wed, Aug 27, 2014 at 2:07 PM, Pavel Stehule <pavel.steh...@gmail.com> > wrote: > >> Hi > >> > >> one user asked about using a partitioning for faster aggregates queries. > >> > >> I found so there is not any optimization. > >> > >> create table x1(a int, d date); > >> create table x_1 ( check(d = '2014-01-01'::date)) inherits(x1); > >> create table x_2 ( check(d = '2014-01-02'::date)) inherits(x1); > >> create table x_3 ( check(d = '2014-01-03'::date)) inherits(x1); > >> > >> When I have this schema, then optimizer try to do > >> > >> postgres=# explain verbose select max(a) from x1 group by d order by d; > >> QUERY PLAN > >> > -------------------------------------------------------------------------------- > >> GroupAggregate (cost=684.79..750.99 rows=200 width=8) > >> Output: max(x1.a), x1.d > >> Group Key: x1.d > >> -> Sort (cost=684.79..706.19 rows=8561 width=8) > >> Output: x1.d, x1.a > >> Sort Key: x1.d > >> -> Append (cost=0.00..125.60 rows=8561 width=8) > >> -> Seq Scan on public.x1 (cost=0.00..0.00 rows=1 > width=8) > >> Output: x1.d, x1.a > >> -> Seq Scan on public.x_1 (cost=0.00..31.40 rows=2140 > >> width=8) > >> Output: x_1.d, x_1.a > >> -> Seq Scan on public.x_2 (cost=0.00..31.40 rows=2140 > >> width=8) > >> Output: x_2.d, x_2.a > >> -> Seq Scan on public.x_3 (cost=0.00..31.40 rows=2140 > >> width=8) > >> Output: x_3.d, x_3.a > >> -> Seq Scan on public.x_4 (cost=0.00..31.40 rows=2140 > >> width=8) > >> Output: x_4.d, x_4.a > >> Planning time: 0.333 ms > >> > >> It can be reduced to: > >> > >> sort by d > >> Append > >> Aggegate (a), d > >> seq scan from x_1 > >> Aggregate (a), d > >> seq scan from x_2 > >> > >> Are there some plans to use partitioning for aggregation? > > > > Besides min/max, what other aggregates (mean/stddev come to mind) > > would you optimize and how would you determine which ones could be? > > Where is that decision made? > > > You can't with mean and stddev, only with associative aggregates. > > That's min, max, sum, bit_and, bit_or, bool_and, bool_or, count. >
I don't think I have a partitions by X .. and my query has group by clause GROUP BY X so I can calculate any aggregate Pavel
