Re: Incomprehensible dogged sort in Merge Join

[Aleksandr Vinokurov <aleksandr(dot)vin(at)gmail(dot)com>]

Gregory Stark wrote:
> "Aleksandr Vinokurov" <aleksandr(dot)vin(at)gmail(dot)com> writes:
>
> > And that is its plan (attached one is the same, but with costs):
> >
> > > ---------------------------------<cut>--------------------------------<
> >  Merge Left Join
> >    Merge Cond: ("outer".name = "inner".name)
> >    ->  Sort
> >          Sort Key: log.name
> >          ->  Seq Scan on log_example_3 log
> >    ->  Sort
> >          Sort Key: uh.name
> >          ->  Subquery Scan uh
> >                ->  Sort
> >                      Sort Key: name
> >                      ->  Seq Scan on user_history
> > > ---------------------------------<cut>--------------------------------<
>
> What version are you running with?

8.0.1, sorry for missing.

> Incidentally with CVS HEAD I see it working:
>
> postgres=# explain select * from (select * from a order by i) as a right join b on a.i = b.i ;
>                            QUERY PLAN                            
> -----------------------------------------------------------------
>  Merge Right Join  (cost=299.56..675.13 rows=22898 width=16)
>    Merge Cond: (public.a.i = b.i)
>    ->  Sort  (cost=149.78..155.13 rows=2140 width=8)
>          Sort Key: public.a.i
>          ->  Seq Scan on a  (cost=0.00..31.40 rows=2140 width=8)
>    ->  Sort  (cost=149.78..155.13 rows=2140 width=8)
>          Sort Key: b.i
>          ->  Seq Scan on b  (cost=0.00..31.40 rows=2140 width=8)
> (8 rows)
>
>
>
> > With best regards to all of you,
> > Aleksandr.
> > ÿþ 
>
> gesundheit.

Best wishes,
Aleksandr.