public void addSampleNulls(ColumnVector col) {
    col.noNulls = false;
    assert col.isNull != null;
    for(int i = 0; i != col.isNull.length; i++) {
      col.isNull[i] = i % 11 == 0;
    }
  }
Add null values, but do it faster, by avoiding use of Random().

@param col
ql/src/test/org/apache/hadoop/hive/ql/exec/vector/TestVectorizedRowBatch.java