Re: slow array(subselect)

2018-03-04 21:36 GMT+01:00 Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>:

>
>
> On 03/04/2018 09:19 PM, Pavel Stehule wrote:
> > Hi
> >
> > Just curious
> >
> > postgres=# explain analyze select array_upper(array_agg(i),1) from
> > generate_series(1,100000) g(i);
> > QUERY
> > PLAN
> > ------------------------------------------------------------
> ---------------------------------------------------------------------
> > Aggregate (cost=12.50..12.52 rows=1 width=4) (actual
> > time=43.164..43.164 rows=1 loops=1)
> > -> Function Scan on generate_series g (cost=0.00..10.00 rows=1000
> > width=4) (actual time=19.150..31.083 rows=100000 loops=1)
> > Planning time: 0.100 ms
> > Execution time: 44.392 ms
> > (4 rows)
> >
> > postgres=# explain analyze select array_upper(array(select i from
> > generate_series(1,1000000) g(i)),1);
> > QUERY
> > PLAN
> > ------------------------------------------------------------
> --------------------------------------------------------------------------
> > Result (cost=10.00..10.01 rows=1 width=4) (actual
> > time=334.567..334.568 rows=1 loops=1)
> > InitPlan 1 (returns $0)
> > -> Function Scan on generate_series g (cost=0.00..10.00 rows=1000
> > width=4) (actual time=141.503..254.483 rows=1000000 loops=1)
> > Planning time: 0.171 ms
> > Execution time: 339.057 ms
> > (5 rows)
> >
> > This code is about 8times slower
> >
>
> Because you're passing 100000 in the first query and 1000000 in the
> second one. So it should be about 10x slower, more or less.
>

true. No miracle. I am tululum :/

Thank you

Pavel

>
> regards
>
> --
> Tomas Vondra http://www.2ndQuadrant.com
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>