@Override
  public void evaluate(VectorizedRowBatch batch) throws HiveException {

    if (childExpressions != null) {
      super.evaluateChildren(batch);
    }

    ColumnVector inputColVector = batch.cols[colNum];
    int[] sel = batch.selected;
    boolean[] nullPos = inputColVector.isNull;
    int n = batch.size;
    if (n <= 0) {
      // Nothing to do
      return;
    }

    if (inputColVector.noNulls) {
      // All selected, do nothing
      return;
    } else if (inputColVector.isRepeating) {
      if (nullPos[0]) {
        // All are null so none are selected
        batch.size = 0;
        return;
      } else {
        // None are null, so all are selected
        return;
      }
    } else if (batch.selectedInUse) {
      int newSize = 0;
      for (int j = 0; j != n; j++) {
        int i = sel[j];
        if (!nullPos[i]) {
          sel[newSize++] = i;
        }
      }
      batch.size = newSize;
    } else {
      int newSize = 0;
      for (int i = 0; i != n; i++) {
        if (!nullPos[i]) {
          sel[newSize++] = i;
        }
      }
      if (newSize < n) {
        batch.selectedInUse = true;
        batch.size = newSize;
      }
    }
  }
This expression selects a row if the given column is null.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/SelectColumnIsNotNull.java