public GenericUDF flip() {
    return this;
  }
Some functions like comparisons may be affected by appearing order of arguments.
This is to convert a function, such as 3 &gt; x to x &lt; 3. The flip function of
GenericUDFOPGreaterThan is GenericUDFOPLessThan.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDF.java