A little background - I have various multi-column indexes whenever I have queries which restrict the output based on the values of the 2 fields (for example, a client code and the date of a transaction).
Is there a performance gain using this approach as opposed to using 2 separate indexes, one on the first column and one on the second column? The reason I am asking is that my coding convetion goes back to the days where I used ISAM tables, so the systems did not know how to use more than a single index. In some cases, I may have an index on (columna, columnb) and one on (columnb, columna) due to the data access patterns. If there are no performance gains in having these multi-part indexes, and performance will be the same as having one index solely on columna and one solely on columnb, then I can reduce the disk usage significantly in some cases.
