public void addRHSSemijoin(String alias) {
    if (!rhsSemijoin.containsKey(alias)) {
      rhsSemijoin.put(alias, null);
    }
  }
Insert only a key to the semijoin table name to column names map.

@param alias
         table name alias.
ql/src/java/org/apache/hadoop/hive/ql/parse/QBJoinTree.java