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.
>
But let's not forget one of the wonders of PostgreSQL.  In this world
you can have your cake and eat it to.  So, create all 4 indices and let
the optimizer sort em out for your queries.

        -DEJ