public void setConstant(ExprNodeConstantDesc expr) {
    isConstant = true;
    if (expr != null) {
      if (expr.getTypeInfo().getCategory() == Category.PRIMITIVE) {
        PrimitiveCategory primitiveCategory = ((PrimitiveTypeInfo) expr.getTypeInfo())
            .getPrimitiveCategory();
        if (primitiveCategory == PrimitiveCategory.VOID) {
          // Otherwise we'd create a NullWritable and that isn't what we want.
          expr = null;
        }
      }
    }

    constExpr = expr;
  }
Set this argument to a constant value extracted from the
expression tree.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/udf/VectorUDFArgDesc.java