>I tried your suggestions but sorry to say that it is still not working. Looks like, it is still in the last (Not at the proper indexed place).
>
>Thanks for your suggestions.

I just had another look at your code:
>Select a
>set rela to x to b
>index on (eof(b),a.x,b.x1) to xidx

Do you mean:

index on IIF( eof(b), a.x, b.x1) to xidx

If so that explains why I didn't understand your index expression.

Why not try:

If EOF()
    Select a
    Index on x to xidx
Else
    Select b
    Index on x to xidx
Endif

Locate

You need to be in the subject table before you can index it.