elation is first scanned
        and loaded into a hash table, using its join attributes as hash keys.
        Next the left relation is scanned and the
        appropriate values of every row found are used as hash keys to
        locate the matching rows in the table.
       </p></li></ul></div><p>
    </p><p>
     When the query involves more than two relations, the final result
     must be built up by a tree of join steps, each with two inputs.
     The planner examines different possible join sequence