@Override
    public void aggregateInputSelection(
      VectorAggregationBufferRow[] aggregationBufferSets,
      int aggregateIndex,
      VectorizedRowBatch batch) throws HiveException {

      int batchSize = batch.size;

      if (batchSize == 0) {
        return;
      }

      inputExpression.evaluate(batch);

      TimestampColumnVector inputVector =
          (TimestampColumnVector)batch.cols[this.inputExpression.getOutputColumnNum()];

      if (inputVector.noNulls) {
        if (inputVector.isRepeating) {
          iterateNoNullsRepeatingWithAggregationSelection(
            aggregationBufferSets, aggregateIndex,
            inputVector.getDouble(0), batchSize);
        } else {
          if (batch.selectedInUse) {
            iterateNoNullsSelectionWithAggregationSelection(
              aggregationBufferSets, aggregateIndex,
              inputVector, batch.selected, batchSize);
          } else {
            iterateNoNullsWithAggregationSelection(
              aggregationBufferSets, aggregateIndex,
              inputVector, batchSize);
          }
        }
      } else {
        if (inputVector.isRepeating) {
          iterateHasNullsRepeatingWithAggregationSelection(
            aggregationBufferSets, aggregateIndex,
            inputVector.getDouble(0), batchSize, inputVector.isNull);
        } else {
          if (batch.selectedInUse) {
            iterateHasNullsSelectionWithAggregationSelection(
              aggregationBufferSets, aggregateIndex,
              inputVector, batchSize, batch.selected, inputVector.isNull);
          } else {
            iterateHasNullsWithAggregationSelection(
              aggregationBufferSets, aggregateIndex,
              inputVector, batchSize, inputVector.isNull);
          }
        }
      }
    }
Value is explicitly (re)initialized in reset()
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/aggregates/VectorUDAFSumTimestamp.java