From: | Robert James <srobertjames(at)gmail(dot)com> |
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To: | Postgres General <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: SORT and Merge Join via Index |
Date: | 2013-08-14 00:04:44 |
Message-ID: | CAGYyBgjW3J7TU5yV=6G2_e_hiEYSourh0ukQnNUXND9HjedgYQ@mail.gmail.com |
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On 8/13/13, Robert James <srobertjames(at)gmail(dot)com> wrote:
> I noticed that when I have an index on (a,b) of table t, and I do an
> SELECT * FROM t ORDER BY a ASC, it doesn't use the index. When I
> create a new index of only a, it does use the index. Why is that?
>
> And, more importantly, when I do a query involving a merge join of
> table t, which requires sorting table t, the planner does the sort
> manually using quicksort, not using the index. The time that step
> takes is identical to the ORDER BY without using the index. What do I
> need to do to have Postgres use the index for the merge join?
>
> (Postgres 8.3)
Interestingly enough, in the JOIN query, if I replace "t" with:
(SELECT f1, f2 FROM t ORDER BY f1 ASC) AS t_
Postgres does use the index, getting the query done in half the time!
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