Optimizing query

Hi! I'm having trouble in a simple query who is not being optimized
the way it should (apparently). The query is as follows:

SELECT DISTINCT e.e_tipo from requisicaoanalise ra, exame e where
ra.ra_reqnum='20010901798' and e.e_req=ra.ra_id;

table requisicaoanalise has +- 10000 rows, and exame hash +- 100000;

This query takes a lot of time (+- 5 secs on my system: psql
(PostgreSQL) 7.1RC3). I have indexes in ra_reqnum as in e_req and ra_id.
What I think is happening here is that postgres 1st evaluates
"e.e_req=ra.ra_id" ans only then "ra.ra_reqnum='20010901798", which is
obviously more time-consuming. If I rewrite the query as

SELECT DISTINCT e_tipo from exame e where e_req in ( select ra_id from
requisicaoanalise where ra_reqnum='20010901798');

the result comes instantly.

Is there any way to specify the precedence of the conditions in the
prior query? I don't have much experienece with the EXPLAIN, but here it
goes:

explain SELECT DISTINCT e.e_tipo from requisicaoanalise ra, exame e
where ra.ra_reqnum='20010901798' and e.e_req=ra.ra_id;
NOTICE: QUERY PLAN:

Unique (cost=22662.94..22867.50 rows=8182 width=12)
-> Sort (cost=22662.94..22662.94 rows=81821 width=12)
-> Merge Join (cost=13756.59..15219.60 rows=81821 width=12)
-> Sort (cost=64.94..64.94 rows=70 width=4)
-> Index Scan using requisicaoanalise_reqnum_idx on
requisicaoanalise ra (cost=0.00..62.79 rows=70 width=4)
-> Sort (cost=13691.65..13691.65 rows=116971 width=8)
-> Seq Scan on exame e (cost=0.00..2835.71 rows=116971
width=8

EXPLAIN SELECT DISTINCT e_tipo from exame e where e_req in ( select
ra_id from requisicaoanalise where ra_reqnum='20010901798');
NOTICE: QUERY PLAN:

Unique (cost=7358767.76..7359060.18 rows=11697 width=4)
-> Sort (cost=7358767.76..7358767.76 rows=116971 width=4)
-> Seq Scan on exame e (cost=0.00..7348057.26 rows=116971 width=4)
SubPlan
-> Materialize (cost=62.79..62.79 rows=70 width=4)
-> Index Scan using requisicaoanalise_reqnum_idx on
requisicaoanalise (cost=0.00..62.79 rows=70 width=4)

Thanks!

--
Pedro Miguel G. Alves

THINK - Tecnologias de Informação
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