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 LEFT JOIN 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 BY grupo.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) (actual time=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=7607 loops=1)
| -> Seq Scan on epr_ord_horario h (cost=0.00..175.07 rows=7607 width=52) (actual time=20.61..2538.44 rows=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..26.25 rows=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_horario AS 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..0.08 rows=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..0.06 rows=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..0.05 rows=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.