Index Using

hello,

I have in db table with 3 columns... table name is l_model_to_part
columns: i_model_id, i_part_id, i_year

I have index on every column separately and primary key is (i_model_id,
i_part_id, i_year)

when I use: EXPLAIN SELECT * FROM l_model_to_part WHERE i_part_id=234;
QUERY PLAN
----------------------------------------------------------------------------------
Index Scan using index_20 on l_model_to_part (cost=0.00..3.37 rows=10
width=12)
Index Cond: (i_part_id = 234)

when I use: EXPLAIN SELECT * FROM l_model_to_part WHERE i_model_id=234;
QUERY PLAN
---------------------------------------------------------------------
Seq Scan on l_model_to_part (cost=0.00..1400.59 rows=866 width=12)
Filter: (i_model_id = 234)

but, when I use: EXPLAIN SELECT * FROM l_model_to_part WHERE
i_model_id=234 AND i_model_id=456;
QUERY PLAN
-----------------------------------------------------------------------------------
Index Scan using index_50 on l_model_to_part (cost=0.00..41.84
rows=11 width=12)
Index Cond: ((i_model_id = 234) AND (i_model_id = 456))

my question is, why postgres doesn't use index_50 in second query???
when I create this structure in clear db, everything is all right...

thanx, hlavki

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