@Explain(vectorization = Vectorization.SUMMARY, displayName = "MergeJoin Vectorization",
      explainLevels = { Level.DEFAULT, Level.EXTENDED })
  public MergeJoinExplainVectorization getReduceExplainVectorization() {
    if (!getVectorizationExamined()) {
      return null;
    }
    return new MergeJoinExplainVectorization(this);
  }
For now, this class just says in EXPLAIN VECTORIZATION we don't support vectorization of the
Merge Join vertex instead of being silent about it.
ql/src/java/org/apache/hadoop/hive/ql/plan/MergeJoinWork.java