public void unFlatten() {
    isRepeating = preFlattenIsRepeating;
    noNulls = preFlattenNoNulls;
  }
Restore the state of isRepeating and noNulls to what it was
before flattening. This must only be called just after flattening
and then evaluating a VectorExpression on the column vector.
It is an optimization that allows other operations on the same
column to continue to benefit from the isRepeating and noNulls
indicators.
storage-api/src/java/org/apache/hadoop/hive/ql/exec/vector/ColumnVector.java