Re: Index on two columns not used

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Arnaud Lesauvage wrote:
I have two table with a 2-column index on both of them.
In the first table, the first colum of the index is the primary key, the second one is an integer field.
In the second table, the two columns are the primary key.
When I join these two tables, the 2-column index of the first table is not used.
Why does the query planner think that this plan  is better ?

ALTER TABLE geo.subcities_names
ADD CONSTRAINT subcities_names_pkey PRIMARY KEY(subcity_gid, language_id);

CREATE INDEX subcities_gid_language_id
  ON geo.subcities
  USING btree
  (gid, official_language_id);

EXPLAIN ANALYZE
SELECT * FROM geo.subcities sc, geo.subcities_names scn
WHERE sc.gid = scn.subcity_gid AND sc.official_language_id = scn.language_id;

My theory:

There's no additional restrictions besides the join condition, so the system has to scan both tables completely. It chooses to use a full index scan instead of a seq scan to be able to do a merge join. Because it's going to have to scan the indexes completely anyway, it chooses the smallest index which is subcities_pkey.

You'd think that the system could do the merge using just the indexes, and only fetch the heap tuples for matches. If that were the case, using the 2-column index would indeed be a good idea. However, PostgreSQL can't use the values stored in the index to check the join condition, so all the heap tuples are fetched anyway. There was just recently discussion about this on this list: http://archives.postgresql.org/pgsql-performance/2006-09/msg00080.php.

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


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