@Override
  public boolean opAllowedBeforeSortMergeJoin() {
    // If a join occurs before the sort-merge join, it is not useful to convert the the sort-merge
    // join to a mapjoin. It might be simpler to perform the join and then a sort-merge join
    // join. By converting the sort-merge join to a map-join, the job will be executed in 2
    // mapjoins in the best case. The number of inputs for the join is more than 1 so it would
    // be difficult to figure out the big table for the mapjoin.
    return false;
  }
Forward a record of join results.

@throws HiveException
ql/src/java/org/apache/hadoop/hive/ql/exec/JoinOperator.java