private void inferBucketingSorting(List<ExecDriver> mapRedTasks) throws SemanticException {
    for (ExecDriver mapRedTask : mapRedTasks) {

      // For now this only is used to determine the bucketing/sorting of outputs, in the future
      // this can be removed to optimize the query plan based on the bucketing/sorting properties
      // of the outputs of intermediate map reduce jobs.
      if (!mapRedTask.getWork().isFinalMapRed()) {
        continue;
      }

      if (mapRedTask.getWork().getReduceWork() == null) {
        continue;
      }
      Operator<? extends OperatorDesc> reducer = mapRedTask.getWork().getReduceWork().getReducer();

      // uses sampling, which means it's not bucketed
      boolean disableBucketing = mapRedTask.getWork().getMapWork().getSamplingType() > 0;
      BucketingSortingCtx bCtx = new BucketingSortingCtx(disableBucketing);

      // RuleRegExp rules are used to match operators anywhere in the tree
      // RuleExactMatch rules are used to specify exactly what the tree should look like
      // In particular, this guarantees that the first operator is the reducer
      // (and its parent(s) are ReduceSinkOperators) since it begins walking the tree from
      // the reducer.
      Map<SemanticRule, SemanticNodeProcessor> opRules = new LinkedHashMap<SemanticRule, SemanticNodeProcessor>();
      opRules.put(new RuleRegExp("R1", SelectOperator.getOperatorName() + "%"),
          BucketingSortingOpProcFactory.getSelProc());
      // Matches only GroupByOperators which are reducers, rather than map group by operators,
      // or multi group by optimization specific operators
      opRules.put(new RuleExactMatch("R2", new String[]{GroupByOperator.getOperatorName()}),
          BucketingSortingOpProcFactory.getGroupByProc());
      // Matches only JoinOperators which are reducers, rather than map joins, SMB map joins, etc.
      opRules.put(new RuleExactMatch("R3", new String[]{JoinOperator.getOperatorName()}),
          BucketingSortingOpProcFactory.getJoinProc());
      opRules.put(new RuleRegExp("R5", FileSinkOperator.getOperatorName() + "%"),
          BucketingSortingOpProcFactory.getFileSinkProc());
      opRules.put(new RuleRegExp("R7", FilterOperator.getOperatorName() + "%"),
          BucketingSortingOpProcFactory.getFilterProc());
      opRules.put(new RuleRegExp("R8", LimitOperator.getOperatorName() + "%"),
          BucketingSortingOpProcFactory.getLimitProc());
      opRules.put(new RuleRegExp("R9", LateralViewForwardOperator.getOperatorName() + "%"),
          BucketingSortingOpProcFactory.getLateralViewForwardProc());
      opRules.put(new RuleRegExp("R10", LateralViewJoinOperator.getOperatorName() + "%"),
          BucketingSortingOpProcFactory.getLateralViewJoinProc());
      // Matches only ForwardOperators which are preceded by some other operator in the tree,
      // in particular it can't be a reducer (and hence cannot be one of the ForwardOperators
      // added by the multi group by optimization)
      opRules.put(new RuleRegExp("R11", ".+" + ForwardOperator.getOperatorName() + "%"),
          BucketingSortingOpProcFactory.getForwardProc());
      // Matches only ForwardOperators which are reducers and are followed by GroupByOperators
      // (specific to the multi group by optimization)
      opRules.put(new RuleExactMatch("R12",new String[]{ ForwardOperator.getOperatorName(),
          GroupByOperator.getOperatorName()}),
          BucketingSortingOpProcFactory.getMultiGroupByProc());

      // The dispatcher fires the processor corresponding to the closest matching rule and passes
      // the context along
      SemanticDispatcher disp = new DefaultRuleDispatcher(BucketingSortingOpProcFactory.getDefaultProc(),
          opRules, bCtx);
      SemanticGraphWalker ogw = new PreOrderWalker(disp);

      // Create a list of topop nodes
      ArrayList<Node> topNodes = new ArrayList<Node>();
      topNodes.add(reducer);
      ogw.startWalking(topNodes, null);

      mapRedTask.getWork().getMapWork().getBucketedColsByDirectory().putAll(bCtx.getBucketedColsByDirectory());
      mapRedTask.getWork().getMapWork().getSortedColsByDirectory().putAll(bCtx.getSortedColsByDirectory());
    }
  }
For each map reduce task, if it has a reducer, infer whether or not the final output of the
reducer is bucketed and/or sorted

@param mapRedTasks
@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/optimizer/physical/BucketingSortingInferenceOptimizer.java