void freeOutputColumn(int index) {
      if (initialOutputCol < 0 || !reuseScratchColumns) {
        // This is a test
        return;
      }
      int colIndex = index-initialOutputCol;
      if (colIndex >= 0) {
        usedOutputColumns.remove(index-initialOutputCol);
      }
    }
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