private String getAliasForTableScanOperator(MapWork work,
      TableScanOperator tso) {
    for (Map.Entry<String, Operator<? extends OperatorDesc>> entry : work
        .getAliasToWork().entrySet()) {
      if (entry.getValue() == tso) {
        return entry.getKey();
      }
    }
    return null;
  }
Iterate over all tasks one by one and removes all input paths from task if
conditions as defined in rules match.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/physical/NullScanTaskDispatcher.java