private void evaluateBytes(VectorizedRowBatch vrg) {
    BytesColumnVector cv = (BytesColumnVector) vrg.cols[outputColumnNum];
    cv.isRepeating = true;
    cv.initBuffer();
    if (!isNullValue) {
      cv.isNull[0] = false;
      cv.setVal(0, bytesValue, 0, bytesValueLength);
    } else {
      cv.isNull[0] = true;
      cv.noNulls = false;
    }
  }
Constant is represented as a vector with repeating values.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/DynamicValueVectorExpression.java