What is the leftmost prefix rule for composite indexes?

A composite index on (col_a, col_b, col_c) can only be used when the query filters include the leftmost columns in order. Supported: WHERE col_a = x (1st column). WHERE col_a = x AND col_b = y (1st and 2nd). WHERE col_a = x AND col_b = y AND col_c = z (all). NOT supported: WHERE col_b = y (skipping col_a). WHERE col_c = z (skipping col_a and col_b). Reason: the B-Tree is sorted first by col_a, the