@Override
      public Text evaluate(Text s) {
        if (s == null) {
          return null;
        }
        t.set(s.toString().toLowerCase());
        return t;
      }
Expression to convert a string to lower case.
Extends {@link StringUnaryUDF}.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/StringLower.java