Re: [SQL] Single vs. multiple indexes

> Question: what's the difference between making a single index covering
> multiple fields of the underlying table, and making a separate index
> for each of the fields?
>
> I understand that the semantics are different if I create a UNIQUE
> index: a unique index on a field enforces that every record in the table
> have a different value of that field, but a unique index on a set of
> fields only enforces that the combination of all those fields be unique
> in each record.
>
> Leaving that aside, what are the performance implications? Does one
> structure support fast execution of query types that the other doesn't,
> and if so which one's better for what? Is a single index cheaper to
> update than multiple indexes? Does it save disk space?
>

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.

Hope this answers the question.

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