public void setReuseColumns(boolean reuseColumns) {
      this.reuseScratchColumns = reuseColumns;
    }
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