Re: combination of function to simple query makes query slow

On 10/3/05, jan aerts (RI) <jan(dot)aerts(at)bbsrc(dot)ac(dot)uk> wrote:
> Some more information:
>
> An EXPLAIN of the following query
> my_db=> explain select m1.object1_id, m1.object2_id, m2.object1_id,
> m2.object2_id
> my_db-> from c_mappings m1, c_mappings m2
> my_db-> where m1.object1_id = 16575564
> my_db-> and m2.object1_id in (select aliases_of(m1.object2_id));
> gives:
> QUERY PLAN
> ------------------------------------------------------------------------
> ----------------------------
> Nested Loop (cost=0.00..99746.00 rows=1170281 width=16)
> Join Filter: (subplan)
> -> Index Scan using ind_cmappings_object1_id on c_mappings m1
> (cost=0.00..6.12 rows=2 width=8)
> Index Cond: (object1_id = 16575564)
> -> Seq Scan on c_mappings m2 (cost=0.00..36052.89 rows=1435589
> width=8)
> SubPlan
> -> Result (cost=0.00..0.01 rows=1 width=0)
> (7 rows)
>
> All columns of c_mappings, as well as the columns that are accessed
> through the aliases_of function, as indexed. However, notice how the
> second loop uses a "Seq Scan" instead of an "Index Scan".
> Is there a way to use an index scan on the results of a function?
>
> Thanks,
> jan.
>

what version is your postgres?

what if you make temp table first? something like this:

select * from c_mappings
where object1_id = 16575564
into temp m1;

select m1.object1_id, m1.object2_id, m2.object1_id, m2.object2_id
from m1, c_mappings m2
where m2.object1_id in (select aliases_of(m1.object2_id));

just an idea...

--
regards,
Jaime Casanova
(DBA: DataBase Aniquilator ;)