From: Heikki Linnakangas <heikki@enterprisedb.com>
To: Arnaud Lesauvage <thewild@freesurf.fr>
Cc: pgsql-performance@postgresql.org
Subject: Re: Index on two columns not used
Date: Wed, 18 Oct 2006 12:40:05 +0100
Message-ID: <45361295.9050504@enterprisedb.com> (raw)
In-Reply-To: <45360E25.9010403@freesurf.fr>
References: <45360E25.9010403@freesurf.fr>
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: heikki@enterprisedb.com, thewild@freesurf.fr
Subject: Re: Index on two columns not used
In-Reply-To: <45361295.9050504@enterprisedb.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox