MySQL 5.0 indexes - Unique vs Non Unique

Question

What is the difference between MySQL unique and non-unique index in terms of performance?

Let us say I want to make an index on a combo of 2 columns, and the comb

Answer 1

On a non-unique index that just happens to be unique and a unique index? I'm not sure, but I'd guess not a lot. The optimiser should examine the cardinality of the index and use that (it will always be the number of rows, for a unique index).

As far as a primary key is concerned, probably quite a lot, but it depends which engine you use.

The InnoDB engine (which is used by many people) always clusters rows on the primary key. This means that the PK is essentially combined with the actual row data. If you're doing a lot of lookups by PK (or indeed, range scans etc), this is a Good Thing, because it means that it won't need to fetch as many blocks from the disc.

A non-PK unique index will never be clustered in InnoDB.

On the other hand, some other engines (MyISAM in particular) don't cluster the PK, so the primary key is just like a normal unique index.