@Explain(displayName = "filter mappings", explainLevels = { Level.EXTENDED })
  public Map<Integer, String> getFilterMapString() {
    return toCompactString(filterMap);
  }
@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