Re: [SQL] Single vs. multiple indexes

Bruce Momjian writes:
> Good question.  The optimizer can only use only one index in a query, so
> if you create five indexes, the optimizer will pick the best one to use,
> and disregard the rest.  If you create one index with five fields, it
> will use as much of the index as it can.  If you restrict on the first
> field of the index, it will use only that part of the index.  If you
> restrict on the first three fields, it will use all three parts,
> resulting in better performance than just having an index on the first
> field.
>
> If you restrict only on the second field of the index, the index is
> useless and will not be used.
>

Just to be sure I understand what you're saying here, let's assume
I've got a table with three integer fields a, b, and c.  I can do
one of two things:

1. Create an index on (a), an index on (b), and an index on (c).
2. Create an index on (a, b, c).

Now let's assume I perform a query that references b only.  In this
case, if I understand you correctly, method #1 above is better,
because the index in method #2 will not be used.

However, if I perform a query that references a and b, then method
#2 would be better, because the composite index will be used,
whereas in method #1, only one of the two relevant indices would be
chosen?


--Jeff