public static SemanticNodeProcessor getGroupByProc() {
    return new MapJoinFollowedByGroupByProcessor();
  }
Node processor factory for map join resolver. What it did is to replace the
map-join operator in the local map join task with a hash-table dump operator.
And if the map join is followed by a group by, the hash-table sink
operator/mapjoin operator should be configured to use less memory to avoid
OOM in group by operator.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/physical/LocalMapJoinProcFactory.java