>I have a parent table P2 and two related child tables C1 and C2.
>Each table has in excess of 1 million records. After SETting
>the RELATIONs, how can I examine only those records in the
>children tables that should be constrained to the parent relationship?
>
>Based on the child related info, I will update a field in the
>parent table.

Assuming you have shared access,
SELECT P2
SCAN FOR C1.fieldname = xxxx
REPLACE .... do whatever
ENDSCAN
SCAN FOR C2.fieldname = xxxx
REPLACE .... do whatever
ENDSCAN

If not multiuser
REPLACE FOR C1.fieldname = xxxx P2field WITH newvalue
will be faster
HTH