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List:       pgsql-performance
Subject:    Re: [PERFORM] Composite keys
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2011-10-31 18:59:19
Message-ID: 5180.1320087559 () sss ! pgh ! pa ! us
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Claudio Freire <klaussfreire@gmail.com> writes:
> On Mon, Oct 31, 2011 at 2:08 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>> Multicolumn indices on (c1, c2, ..., cn) can only be used on where
>>> clauses involving c1..ck with k<n.

>> I don't think that's true.  I believe it can be used for a query that
>> only touches, say, c2.  It's just extremely inefficient.

> Does postgres generate those kinds of plans?

Sure it does.  It doesn't usually think they're efficient enough,
because they require full-index scans.  But sometimes that's the
best you can do.

			regards, tom lane

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