Re: index-only scans

On Aug 12, 2011, at 10:03 PM, Heikki Linnakangas wrote:

> 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)

is there any plan to revise the cost for index only scans compared to what it is now?

many thanks,

hans

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