Re: If an index is based on 3 columns will a query using two of the columns utilize the index?

Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> On Mon, Sep 12, 2005 at 09:43:57AM -0400, Reid Thompson wrote:
>> Example:
>> assume a table of 10 columns, three of which are fname, lname, and dob.
>> If an index is created on (fname, lname, dob), will a query that
>> utilizes two of the columns ( select 'data' from table where fname = 'X'
>> and lname = 'Y') utilize the index?

> Yes, if it is selective enough.  (It _can_ use the index, which does not
> mean that it _will_ use it.)  Note that if your example query used the
> columns (lname, dob), the answer would be "no."

Actually, that last point is not true anymore as of 8.1 --- see this
thread:
http://archives.postgresql.org/pgsql-hackers/2005-05/msg00939.php
which led to this patch:
http://archives.postgresql.org/pgsql-committers/2005-06/msg00156.php

I missed the fact that the documentation said it wouldn't work though.
Will fix...

            regards, tom lane