Point of combined indexes

[bauhsoj]

What is the point of combined (multi-column) indexes and how can I best use them to speed up queries under different circumstances?

[Fumigator]

A compound index is used to make a unique primary key on a table when it doesn't make any sense to create another column whose only purpose is to provide a unique primary key.

They are used just like any other index is used, but the order of the two columns matters. For example, a query that uses the second column in the index but not the first column will probably not use that index in the access path.