private int[][] compactFilter(int[][] filterMap) {
    if (filterMap == null) {
      return null;
    }
    for (int i = 0; i < filterMap.length; i++) {
      if (filterMap[i] != null) {
        boolean noFilter = true;
        // join positions for even index, filter lengths for odd index
        for (int j = 1; j < filterMap[i].length; j += 2) {
          if (filterMap[i][j] > 0) {
            noFilter = false;
            break;
          }
        }
        if (noFilter) {
          filterMap[i] = null;
        }
      }
    }
    for (int[] mapping : filterMap) {
      if (mapping != null) {
        return filterMap;
      }
    }
    return null;
  }
@param skewKeysValuesTables
         set the table desc for storing skew keys and their corresponding
         value;
ql/src/java/org/apache/hadoop/hive/ql/plan/JoinDesc.java