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?

Exactly.

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