left join query does not perform well

Hi all,

I have three tables that are related in the following way:

'----------------------------------------
| epr_ord_grupo < 1 - n > epr_ord_profesor < 0 - n > epr_ord_horario
`----------------------------------------

In average there is 1.0407716514 rows in epr_ord_horario for each row
in epr_ord_profesor, but since there may be 0 I use the query
explained below:

'----------------------------------------
| test=# explain analyze SELECT grupo.grupo_id FROM (epr_vord_grupo AS grupo INNER JOIN (epr_vord_profesor AS profesor
LEFTJOIN epr_vord_horario AS horario ON (profesor.profesor_id = horario.profesor_id)) ON (grupo.grupo_id =
profesor.grupo_id))WHERE (grupo.año_semestre = cast('20022' as integer)) AND (grupo.grupo = cast('4021' as int)) ORDER
BYgrupo.grupo_id, profesor.cargo, profesor.profesor_id, horario.horario_id; 
| NOTICE:  QUERY PLAN:
|
| Sort  (cost=1165.25..1165.25 rows=2 width=24) (actual time=3113.30..3113.31 rows=3 loops=1)
|   ->  Hash Join  (cost=670.71..1165.24 rows=2 width=24) (actual time=3016.64..3113.08 rows=3 loops=1)
|         ->  Merge Join  (cost=665.46..1110.66 rows=9859 width=20) (actual time=2774.61..3058.77 rows=10157 loops=1)
|               ->  Index Scan using epr_ord_profesor_pkey on epr_ord_profesor  (cost=0.00..306.45 rows=9859 width=12)
(actualtime=124.07..275.04 rows=9859 loops=1) 
|               ->  Sort  (cost=665.46..665.46 rows=7607 width=52) (actual time=2650.49..2660.55 rows=7607 loops=1)
|                     ->  Subquery Scan horario  (cost=0.00..175.07 rows=7607 width=52) (actual time=20.62..2593.15
rows=7607loops=1) 
|                           ->  Seq Scan on epr_ord_horario h  (cost=0.00..175.07 rows=7607 width=52) (actual
time=20.61..2538.44rows=7607 loops=1) 
|         ->  Hash  (cost=5.25..5.25 rows=1 width=4) (actual time=26.26..26.26 rows=0 loops=1)
|               ->  Index Scan using llave primaria-asemg on epr_ord_grupo g  (cost=0.00..5.25 rows=1 width=4) (actual
time=26.24..26.25rows=1 loops=1) 
| Total runtime: 3117.48 msec
|
| EXPLAIN
`----------------------------------------

However the full join (which is correct almost always) of the three
tables looks like:

'----------------------------------------
| test=# explain analyze SELECT grupo.grupo_id FROM epr_vord_grupo grupo, epr_vord_profesor AS profesor,
epr_vord_horarioAS horario WHERE  (profesor.profesor_id = horario.profesor_id) AND (grupo.grupo_id = profesor.grupo_id)
AND(grupo.año_semestre = cast('20022' as integer)) AND (grupo.grupo = cast('4021' as int)) ORDER BY grupo.grupo_id,
profesor.cargo,profesor.profesor_id, horario.horario_id; 
| NOTICE:  QUERY PLAN:
|
| Sort  (cost=14.91..14.91 rows=2 width=24) (actual time=0.53..0.53 rows=3 loops=1)
|   ->  Nested Loop  (cost=0.00..14.90 rows=2 width=24) (actual time=0.19..0.30 rows=3 loops=1)
|         ->  Nested Loop  (cost=0.00..8.48 rows=2 width=16) (actual time=0.14..0.17 rows=2 loops=1)
|               ->  Index Scan using llave primaria-asemg on epr_ord_grupo g  (cost=0.00..5.25 rows=1 width=4) (actual
time=0.08..0.08rows=1 loops=1) 
|               ->  Index Scan using epr_ord_profe_gpo on epr_ord_profesor  (cost=0.00..3.20 rows=2 width=12) (actual
time=0.04..0.06rows=2 loops=1) 
|         ->  Index Scan using epr_ord_horario_prf on epr_ord_horario h  (cost=0.00..3.04 rows=1 width=8) (actual
time=0.03..0.05rows=2 loops=2) 
| Total runtime: 1.15 msec
|
| EXPLAIN
`----------------------------------------

Ouch, 3117.48 msec vs. 1.15 msec is a huge difference. I need
something else? or may be postgres optimizer can't cope with
left/right joins?

I was thinking to add a dummy row in epr_ord_horario to ensure that
the full join is always correct but may be a better solution arises
from the expertise of the list :-)

Regards,
Manuel.