Hi all!
I have such table:
CREATE TABLE object ( id SERIAL, object_type_id int8 );
This table has 4 000 000 rows.
There are 2 index: CREATE INDEX object_id_idx ON object(id); CREATE INDEX object_object_type_id_idx ON object(object_type_id);
So:
# EXPLAIN SELECT * FROM object WHERE id = 1::int8;
Index Scan using object_id_idx on object (cost=0.00..92323.66 rows=23650 width=29)
Index Cond: (id = 1::bigint)
Here everything is O.K.
# EXPLAIN SELECT * FROM object WHERE object_type_id = 1::int8;
Index Scan using object_object_type_id_idx on object (cost=0.00..92323.66 rows=23650 width=29)
Index Cond: (object_type_id = 1::bigint)
Here everything is O.K. too... but!
# EXPLAIN SELECT * FROM object WHERE object_type_id IN (1::int8, 21::int8); Seq Scan on object (cost=0.00..105730.00 rows=47182 width=29) Filter: ((object_type_id = 1::bigint) OR (object_type_id = 21::bigint))
The same results after:
# EXPLAIN SELECT * FROM object WHERE object_type_id = 1::int8 OR object_type_id = 21::int8;
Why Postgres didn't use index here ???
# EXPLAIN ANALYZE SELECT * FROM object WHERE object_type_id IN (1::int8, 21::int8);
Seq Scan on object (cost=0.00..105730.00 rows=47182 width=29) (actual time=20744.910..20744.910 rows=0 loops=1)
Filter: ((object_type_id = 1::bigint) OR (object_type_id = 21::bigint))
Total runtime: 20745.022 ms
Best regards, Vladimir S. Tikhonjuk
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