This is a complex read-only transaction. This finds all distinct items which have been ordered within the last n orders from a district having a stock level lower than a given threshold. The SQL statement:

        SELECT
            COUNT (DISTINCT S_I_ID) INTO N_ITEMS
        FROM
            ORDER_LINE, STOCK
        WHERE
                OL_W_ID = W_ID
            AND OL_D_ID = D_ID
            AND OL_O_ID < LAST_O
            AND OL_O_ID >= LAST_O - 20
            AND S_W_ID = W_ID
            AND S_I_ID = OL_I_ID
            AND S_QUANTITY < THRESHOLD;

is the fastest way of getting this. Note that the ORDER_LINE is the driving table of join (leftmost in FROM). Also not the use of historical read in the client (SQL_CONCURRENCY option).