[jiggawatts]

Skip-scan or similar code can solve the problem if you have a compound index on both columns.

The query planner can include the entire matching range for the 'A' column and check the 'B' column for matches via skip-scan.

This is possible in principle, but I don't believe most (any?) database engines use this specific approach.

It could be optimal if the results are required in A,B sorted order.