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Date: Wed, 18 Oct 2006 12:40:05 +0100
From: "Heikki Linnakangas" <heikki@enterprisedb.com>
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To: "Arnaud Lesauvage" <thewild@freesurf.fr>
Cc: <pgsql-performance@postgresql.org>
Subject: Re: Index on two columns not used
References: <45360E25.9010403@freesurf.fr>
In-Reply-To: <45360E25.9010403@freesurf.fr>
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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