Why the planner is not using the INDEX .

Hi all,

If you can just help my understanding the choice of the planner.

Here is the Query:
explain analyse SELECT IRNUM FROM IR
INNER JOIN IT ON IT.ITIRNUM = ANY ('{1000, 2000}') AND
IT.ITYPNUM = 'M' AND IR.IRYPNUM = IT.ITYPNUM AND IR.IRNUM = IT.ITIRNUM
WHERE IRNUM = ANY ('{1000, 2000}') and IRYPNUM = 'M'

Here is the Query plan:

QUERY PLAN






Hash Join (cost=1142.47..5581.75 rows=87 width=4) (actual
time=125.000..203.000 rows=2 loops=1)
Hash Cond: ("outer".itirnum = "inner".irnum)




-> Seq Scan on it (cost=0.00..3093.45 rows=31646 width=9) (actual
time=0.000..78.000 rows=2 loops=1)
Filter: ((itirnum = ANY ('{1000,2000}'::integer[])) AND
((itypnum)::text = 'M'::text))

-> Hash (cost=1142.09..1142.09 rows=151 width=37) (actual
time=125.000..125.000 rows=0 loops=1)
-> Index Scan using ir_pk on ir (cost=0.00..1142.09 rows=151
width=37) (actual time=0.000..125.000 rows=2 loops=1)
Index Cond: ((irypnum)::text = 'M'::text)




Filter: (irnum = ANY ('{1000,2000}'::integer[]))




Total runtime: 203.000 ms

I don't understand why the planner do a Seq Scan (Seq Scan on table
IT ..) instead of passing by the followin index:
ALTER TABLE IT ADD CONSTRAINT IT_IR_FK foreign key (ITYPNUM,ITIRNUM)
references IR (IRYPNUM, IRNUM) ON UPDATE CASCADE;

I tried some stuff but I'm not able to change this behavior. The IT and
IR table may be quite huge (from 20k to 1600k rows) so I think doing a
SEQ SCAN is not a good idea.. am I wrong? Is this query plan is oki for
you ?

Thanks for your help.

/David
P.S.: I'm using postgresql 8.0.3 on windows and I change those setting
in my postgresql.conf :
shared_buffers = 12000 # min 16, at least max_connections*2, 8KB each
work_mem = 15000 # min 64, size in KB