>>fld1 = x and fld2 = y

In this case, you need two indexes for FULL optimization. One of the expressions needs to be on fld1 and the other on fld2. Also, if Set("deleted") = "ON", you also need an index on the expression DELETED().

You can go with partial optimiztion, which usually gets you 80% of the way there anyway with only 1 index.