@VisibleForTesting
  public void finishedAddingInitialColumns() {
    int firstOutputColumnIndex = projectedColumns.size();
    this.ocm = new OutputColumnManager(firstOutputColumnIndex);
    this.ocm.setReuseColumns(this.reuseScratchColumns);
    this.firstOutputColumnIndex = firstOutputColumnIndex;
  }
Context class for vectorization execution.
Main role is to map column names to column indices and serves as a
factory class for building vectorized expressions out of descriptors.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/VectorizationContext.java