SET HASHJOIN allows the query optimizer to consider using hash joins.

SET NOHASHJOIN stops the query optimizer from considering the use of hash joins.

A hash join is one in which a hash table is built with the rows of one of the join sources by hashing on the key columns of the join. The rows of the other join source are then read and hashed into the table on their key columns. The hashing of the second set of rows should quickly identify pairs of joining rows. The advantage of this technique is that it requires no index structures on the join columns (as does KEY join), nor does it require sorting on the join columns (as does merge join).

SET HASHJOIN is the default.