public void markScratchColumns() {
      if (markedScratchColumns != null) {
        throw new RuntimeException("Illegal call");
      }
      markedScratchColumns = Arrays.copyOf(scratchColumnTrackWasUsed, scratchColumnTrackWasUsed.length);
    }
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