Re: index-only scans

On 11.08.2011 23:06, Robert Haas wrote:
> Comments, testing, review appreciated...

I would've expected this to use an index-only scan:

postgres=# CREATE TABLE foo AS SELECT generate_series(1,100000) AS id;
SELECT 100000
postgres=# CREATE INDEX i_foo ON foo (id) WHERE id = 10;
CREATE INDEX
postgres=# VACUUM ANALYZE foo;
VACUUM
postgres=# EXPLAIN SELECT id FROM foo WHERE id = 10;
QUERY PLAN
-----------------------------------------------------------------
Index Scan using i_foo on foo (cost=0.00..8.27 rows=1 width=4)
Index Cond: (id = 10)
(2 rows)

If it's not a predicate index, then it works:

postgres=# DROP INDEX i_foo;
DROP INDEX
postgres=# EXPLAIN SELECT id FROM foo WHERE id = 10;
QUERY PLAN
-----------------------------------------------------------------------
Index Only Scan using i_foo2 on foo (cost=0.00..8.28 rows=1 width=4)
Index Cond: (id = 10)
(2 rows)

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com