public final int decRef() {
    int i = refCount.decrementAndGet();
    assert i >= 0;
    return i;
  }
Resets the column to default state
 - fills the isNull array with false
 - sets noNulls to true
 - sets isRepeating to false
storage-api/src/java/org/apache/hadoop/hive/ql/exec/vector/ColumnVector.java