From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Barker <mikeb01@gmail.com>
cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Query doesn't use index on hstore column
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Comments: In-reply-to Michael Barker <mikeb01@gmail.com>
 message dated "Fri, 05 Dec 2014 09:42:20 +1300"
Date: Thu, 04 Dec 2014 20:32:50 -0500
Message-ID: <20532.1417743170@sss.pgh.pa.us>
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Sender: pgsql-performance-owner@postgresql.org

Michael Barker <mikeb01@gmail.com> writes:
> I'm currently experimenting with hstore on Posgtres 9.4rc1.  I've created a
> table with an hstore column, with and index on that column (tried both gin
> and btree indexes) and the explain plan says that the index is never used
> for the lookup and falls to a sequential scan every time (table has 1 000
> 000 rows).  The query plans and execution time for btree index, gin index
> and unindexed are the same.  Is there something I'm doing wrong or missing
> in order to get indexes to work on hstore columns?

Well, first off, a btree index is fairly useless for this query,
because btree has no concept that the hstore has any sub-structure.
A GIN index or GIST index could work though.  Secondly, you have to
remember that indexable WHERE conditions in Postgres are *always* of
the form "WHERE indexed_column indexable_operator some_comparison_value".
So the trick is to recast the condition you have into something that
looks like that.  Instead of

	WHERE attributes->'accountId' = '1879355460'

you could do

	WHERE attributes @> 'accountId=>1879355460'

(@> being the hstore containment operator, ie "does attributes contain
a pair that looks like this?") or equivalently but possibly easier to
generate,

	WHERE attributes @> hstore('accountId', '1879355460')

Another possibility if you're only concerned about indexing searches
for one or a few specific keys is to use expression indexes:

	CREATE INDEX ON audit ((attributes->'accountId'));

whereupon your original query works, since the left-hand side of
the '=' operator is now the indexed expression.  (Here, since you
are testing plain equality on the indexed value, a btree works fine.)

You might care to read
http://www.postgresql.org/docs/9.4/static/indexes.html
to get a better handle on what Postgres indexes can and can't do.

			regards, tom lane


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