@Override
    protected Set<String> getAliases(Node nd) {
      return getQualifiedAliases((JoinOperator) nd, ((JoinOperator)nd).getSchema());
    }
Determines predicates for which alias can be pushed to it's parents. See
the comments for getQualifiedAliases function.
ql/src/java/org/apache/hadoop/hive/ql/ppd/OpProcFactory.java