private static Set<String> signaturesOf(Set<TableScanOperator> ops) {
    Set<String> ret = new HashSet<>();
    for (TableScanOperator o : ops) {
      ret.add(o.getConf().getQualifiedTable());
    }
    return ret;
  }
Determines if the two trees are using independent inputs.
ql/src/java/org/apache/hadoop/hive/ql/exec/OperatorUtils.java