Re: SORT and Merge Join via Index

On 8/13/13, Robert James <srobertjames@gmail.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!