public void addComparisonOp(String columnName, String... udfs) {
    Set<String> allowed = columnToUDFs.get(columnName);
    if (allowed == null || allowed == udfNames) {
      // override
      columnToUDFs.put(columnName, new HashSet<String>(Arrays.asList(udfs)));
    } else {
      allowed.addAll(Arrays.asList(udfs));
    }
  }
add allowed functions per column
@param columnName
@param udfs
ql/src/java/org/apache/hadoop/hive/ql/index/IndexPredicateAnalyzer.java