Your rule is essentially correct.

You said your index is INDEX ON A+B TAG SOMETAG

You SELECT commands won't use this tag as they stand. The SQL commands you wrote would benefit from indexes like these...

INDEX ON A TAG ATAG
INDEX ON B TAG BTAG

In order to do an SQL command that uses the first index, you must...

SELECT WHERE A+B = vp_A+vp_B.

If you try ...

SELECT WHERE B+A = vp_B+vp_A. This would be slow because it isn't using any of the indexes.

>Example:
>If my index on a table is A + B
>(two chacter based fields to keep things simple) and I then have
>
>SELECT statement with WHERE B = vp_B AND A = vp_A
>
>will this run at the same speed as the same as
>
>SELECT statement but with WHERE A = vp_A AND B = vp_B
>
>Thanks
>Jon