Re: Help with rewriting query - Mailing list pgsql-performance

Hi Bruno,
I followed your suggestion.
The query plan shows that it uses the index (id, person_id). However,
the execution time is still slow. I have to do ctl-C to stop it.
Maybe something is wrong with my postgresql config.
It's running Solaris on dual Opteron, 4GB.
I allocated around 128MB for sorting and more than 80% for
effective_cache_size and shared_buffers = 32768.
Any further ideas is much appreciated.




On 6/8/05, Bruno Wolff III <bruno@wolff.to> wrote:
> On Wed, Jun 08, 2005 at 15:48:27 -0700,
>  Junaili Lie <junaili@gmail.com> wrote:
> > Hi,
> > The suggested query below took forever when I tried it.
> > In addition, as suggested by Tobias, I also tried to create index on
> > food(p_id, id), but still no goal (same query plan).
> > Here is the explain:
> > TEST1=# explain select f.p_id, max(f.id) from Food f, Person p where
> > (f.p_id = p.id) group by p.id;
>
> The above is going to require reading all the food table (assuming no
> orphaned records), so the plan below seems reasonable.
>
> >                                                   QUERY PLAN
> > ----------------------------------------------------------------------------------------------------------------
> >  GroupAggregate  (cost=0.00..214585.51 rows=569 width=16)
> >   ->  Merge Join  (cost=0.00..200163.50 rows=2884117 width=16)
> >         Merge Cond: ("outer".id = "inner".p_id)
> >         ->  Index Scan using person_pkey on person p
> > (cost=0.00..25.17 rows=569 width=8)
> >         ->  Index Scan using person_id_food_index on food f
> > (cost=0.00..164085.54 rows=2884117 width=16)
> > (5 rows)
> >
> >
> >
> >
> > TEST1=# explain select p.id, (Select f.id from food f where
> > f.p_id=p.id order by f.id desc limit 1) from person p;
>
> Using a subselect seems to be the best hope of getting better performance.
> I think you almost got it right, but in order to use the index on
> (p_id, id) you need to order by f.p_id desc, f.id desc. Postgres won't
> deduce this index can be used because f.p_id is constant in the subselect,
> you need to give it some help.
>
> >                                                QUERY PLAN
> > -----------------------------------------------------------------------------------------------------------
> >  Seq Scan on Person p  (cost=100000000.00..100007015.24 rows=569 width=8)
> >   SubPlan
> >     ->  Limit  (cost=0.00..12.31 rows=1 width=8)
> >           ->  Index Scan Backward using food_pkey on food f
> > (cost=0.00..111261.90 rows=9042 width=8)
> >                 Filter: (p_id = $0)
> > (5 rows)
> >
> > any ideas or suggestions is appreciate.
> >
> >
> > On 6/8/05, Tobias Brox <tobias@nordicbet.com> wrote:
> > > [Junaili Lie - Wed at 12:34:32PM -0700]
> > > > select f.p_id, max(f.id) from person p, food f where p.id=f.p_id group
> > > > by f.p_id will work.
> > > > But I understand this is not the most efficient way. Is there another
> > > > way to rewrite this query? (maybe one that involves order by desc
> > > > limit 1)
> > >
> > > eventually, try something like
> > >
> > >  select p.id,(select f.id from food f where f.p_id=p.id order by f.id desc limit 1)
> > >  from person p
> > >
> > > not tested, no warranties.
> > >
> > > Since subqueries can be inefficient, use "explain analyze" to see which one
> > > is actually better.
> > >
> > > This issue will be solved in future versions of postgresql.
> > >
> > > --
> > > Tobias Brox, +47-91700050
> > > Tallinn
> > >
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 9: the planner will ignore your desire to choose an index scan if your
> >       joining column's datatypes do not match
>