@Override
    public Object process(
        Node nd, Stack<Node> stack, NodeProcessorCtx procCtx, Object... nodeOutputs)
        throws SemanticException {

      // If the reduce sink has not been introduced due to bucketing/sorting, ignore it
      FileSinkOperator fsOp = (FileSinkOperator) nd;
      ReduceSinkOperator rsOp =
          (ReduceSinkOperator) fsOp.getParentOperators().get(0).getParentOperators().get(0);

      List<ReduceSinkOperator> rsOps =
          pGraphContext.getReduceSinkOperatorsAddedByEnforceBucketingSorting();
      // nothing to do
      if ((rsOps != null) && (!rsOps.contains(rsOp))) {
        return null;
      }

      // Don't do this optimization with updates or deletes
      if (pGraphContext.getContext().getAcidOperation() == AcidUtils.Operation.UPDATE
          || pGraphContext.getContext().getAcidOperation() == AcidUtils.Operation.DELETE) {
        return null;
      }

      if (stack.get(0) instanceof TableScanOperator) {
        TableScanOperator tso = ((TableScanOperator) stack.get(0));
        if (SemanticAnalyzer.isAcidTable(tso.getConf().getTableMetadata())) {
          /*ACID tables have complex directory layout and require merging of delta files
           * on read thus we should not try to read bucket files directly*/
          return null;
        }
      }
      // Support for dynamic partitions can be added later
      if (fsOp.getConf().getDynPartCtx() != null) {
        return null;
      }

      // No conversion is possible for the reduce keys
      for (ExprNodeDesc keyCol : rsOp.getConf().getKeyCols()) {
        if (!(keyCol instanceof ExprNodeColumnDesc)) {
          return null;
        }
      }

      Table destTable = fsOp.getConf().getTable();
      if (destTable == null) {
        return null;
      }
      int numBucketsDestination = destTable.getNumBuckets();

      // Get the positions for sorted and bucketed columns
      // For sorted columns, also get the order (ascending/descending) - that should
      // also match for this to be converted to a map-only job.
      // Get the positions for sorted and bucketed columns
      // For sorted columns, also get the order (ascending/descending) - that should
      // also match for this to be converted to a map-only job.
      List<Integer> bucketPositions =
          getBucketPositions(destTable.getBucketCols(), destTable.getCols());
      ObjectPair<List<Integer>, List<Integer>> sortOrderPositions =
          getSortPositionsOrder(destTable.getSortCols(), destTable.getCols());
      List<Integer> sortPositions = sortOrderPositions.getFirst();
      List<Integer> sortOrder = sortOrderPositions.getSecond();
      boolean useBucketSortPositions = true;

      // Only selects and filters are allowed
      Operator<? extends OperatorDesc> op = rsOp;
      // TableScan will also be followed by a Select Operator. Find the expressions for the
      // bucketed/sorted columns for the destination table
      List<ExprNodeColumnDesc> sourceTableBucketCols = new ArrayList<ExprNodeColumnDesc>();
      List<ExprNodeColumnDesc> sourceTableSortCols = new ArrayList<ExprNodeColumnDesc>();
      op = op.getParentOperators().get(0);

      while (true) {
        if (!(op instanceof TableScanOperator)
            && !(op instanceof FilterOperator)
            && !(op instanceof SelectOperator)
            && !(op instanceof SMBMapJoinOperator)) {
          return null;
        }

        if (op instanceof SMBMapJoinOperator) {
          // Bucketing and sorting keys should exactly match
          if (!(bucketPositions.equals(sortPositions))) {
            return null;
          }
          SMBMapJoinOperator smbOp = (SMBMapJoinOperator) op;
          SMBJoinDesc smbJoinDesc = smbOp.getConf();
          int posBigTable = smbJoinDesc.getPosBigTable();

          // join keys dont match the bucketing keys
          List<ExprNodeDesc> keysBigTable = smbJoinDesc.getKeys().get((byte) posBigTable);
          if (keysBigTable.size() != bucketPositions.size()) {
            return null;
          }

          if (!validateSMBJoinKeys(
              smbJoinDesc, sourceTableBucketCols, sourceTableSortCols, sortOrder)) {
            return null;
          }

          sourceTableBucketCols.clear();
          sourceTableSortCols.clear();
          useBucketSortPositions = false;

          for (ExprNodeDesc keyBigTable : keysBigTable) {
            if (!(keyBigTable instanceof ExprNodeColumnDesc)) {
              return null;
            }
            sourceTableBucketCols.add((ExprNodeColumnDesc) keyBigTable);
            sourceTableSortCols.add((ExprNodeColumnDesc) keyBigTable);
          }

          // since it is a sort-merge join, only follow the big table
          op = op.getParentOperators().get(posBigTable);
        } else {
          // nothing to be done for filters - the output schema does not change.
          if (op instanceof TableScanOperator) {
            assert !useBucketSortPositions;
            TableScanOperator ts = (TableScanOperator) op;
            Table srcTable = ts.getConf().getTableMetadata();

            // Find the positions of the bucketed columns in the table corresponding
            // to the select list.
            // Consider the following scenario:
            // T1(key, value1, value2) bucketed/sorted by key into 2 buckets
            // T2(dummy, key, value1, value2) bucketed/sorted by key into 2 buckets
            // A query like: insert overwrite table T2 select 1, key, value1, value2 from T1
            // should be optimized.

            // Start with the destination: T2, bucketed/sorted position is [1]
            // At the source T1, the column corresponding to that position is [key], which
            // maps to column [0] of T1, which is also bucketed/sorted into the same
            // number of buckets
            List<Integer> newBucketPositions = new ArrayList<Integer>();
            for (int pos = 0; pos < bucketPositions.size(); pos++) {
              ExprNodeColumnDesc col = sourceTableBucketCols.get(pos);
              String colName = col.getColumn();
              int bucketPos = findColumnPosition(srcTable.getCols(), colName);
              if (bucketPos < 0) {
                return null;
              }
              newBucketPositions.add(bucketPos);
            }

            // Find the positions/order of the sorted columns in the table corresponding
            // to the select list.
            List<Integer> newSortPositions = new ArrayList<Integer>();
            for (int pos = 0; pos < sortPositions.size(); pos++) {
              ExprNodeColumnDesc col = sourceTableSortCols.get(pos);
              String colName = col.getColumn();
              int sortPos = findColumnPosition(srcTable.getCols(), colName);
              if (sortPos < 0) {
                return null;
              }
              newSortPositions.add(sortPos);
            }

            if (srcTable.isPartitioned()) {
              PrunedPartitionList prunedParts =
                  pGraphContext.getPrunedPartitions(srcTable.getTableName(), ts);
              List<Partition> partitions = prunedParts.getNotDeniedPartns();

              // Support for dynamic partitions can be added later
              // The following is not optimized:
              // insert overwrite table T1(ds='1', hr) select key, value, hr from T2 where ds = '1';
              // where T1 and T2 are bucketed by the same keys and partitioned by ds. hr
              if ((partitions == null) || (partitions.isEmpty()) || (partitions.size() > 1)) {
                return null;
              }
              for (Partition partition : partitions) {
                if (!checkPartition(
                    partition,
                    newBucketPositions,
                    newSortPositions,
                    sortOrder,
                    numBucketsDestination)) {
                  return null;
                }
              }

              removeReduceSink(
                  rsOp, (TableScanOperator) op, fsOp, partitions.get(0).getSortedPaths());
              return null;
            } else {
              if (!checkTable(
                  srcTable,
                  newBucketPositions,
                  newSortPositions,
                  sortOrder,
                  numBucketsDestination)) {
                return null;
              }

              removeReduceSink(rsOp, (TableScanOperator) op, fsOp, srcTable.getSortedPaths());
              return null;
            }
          }
          // None of the operators is changing the positions
          else if (op instanceof SelectOperator) {
            SelectOperator selectOp = (SelectOperator) op;
            SelectDesc selectDesc = selectOp.getConf();

            // Iterate backwards, from the destination table to the top of the tree
            // Based on the output column names, get the new columns.
            if (!useBucketSortPositions) {
              bucketPositions.clear();
              sortPositions.clear();
              List<String> outputColumnNames = selectDesc.getOutputColumnNames();

              for (ExprNodeColumnDesc col : sourceTableBucketCols) {
                String colName = col.getColumn();
                int colPos = outputColumnNames.indexOf(colName);
                if (colPos < 0) {
                  return null;
                }
                bucketPositions.add(colPos);
              }

              for (ExprNodeColumnDesc col : sourceTableSortCols) {
                String colName = col.getColumn();
                int colPos = outputColumnNames.indexOf(colName);
                if (colPos < 0) {
                  return null;
                }
                sortPositions.add(colPos);
              }
            }

            // There may be multiple selects - chose the one closest to the table
            sourceTableBucketCols.clear();
            sourceTableSortCols.clear();

            // Only columns can be selected for both sorted and bucketed positions
            for (int pos : bucketPositions) {
              ExprNodeDesc selectColList = selectDesc.getColList().get(pos);
              if (!(selectColList instanceof ExprNodeColumnDesc)) {
                return null;
              }
              sourceTableBucketCols.add((ExprNodeColumnDesc) selectColList);
            }

            for (int pos : sortPositions) {
              ExprNodeDesc selectColList = selectDesc.getColList().get(pos);
              if (!(selectColList instanceof ExprNodeColumnDesc)) {
                return null;
              }
              sourceTableSortCols.add((ExprNodeColumnDesc) selectColList);
            }

            useBucketSortPositions = false;
          }
          op = op.getParentOperators().get(0);
        }
      }
    }
    // The output columns for the destination table should match with the join keys
    // This is to handle queries of the form:
    // insert overwrite table T3
    // select T1.key, T1.key2, UDF(T1.value, T2.value)
    // from T1 join T2 on T1.key = T2.key and T1.key2 = T2.key2
    // where T1, T2 and T3 are bucketized/sorted on key and key2
    // Assuming T1 is the table on which the mapper is run, the following is true:
    // . The number of buckets for T1 and T3 should be same
    // . The bucketing/sorting columns for T1, T2 and T3 should be same
    // . The sort order of T1 should match with the sort order for T3.
    // . If T1 is partitioned, only a single partition of T1 can be selected.
    // . The select list should contain with (T1.key, T1.key2) or (T2.key, T2.key2)
    // . After the join, only selects and filters are allowed.
    private boolean validateSMBJoinKeys(
        SMBJoinDesc smbJoinDesc,
        List<ExprNodeColumnDesc> sourceTableBucketCols,
        List<ExprNodeColumnDesc> sourceTableSortCols,
        List<Integer> sortOrder) {
      // The sort-merge join creates the output sorted and bucketized by the same columns.
      // This can be relaxed in the future if there is a requirement.
      if (!sourceTableBucketCols.equals(sourceTableSortCols)) {
        return false;
      }

      // Get the total number of columns selected, and for each output column, store the
      // base table it points to. For
      // insert overwrite table T3
      // select T1.key, T1.key2, UDF(T1.value, T2.value)
      // from T1 join T2 on T1.key = T2.key and T1.key2 = T2.key2
      // the following arrays are created
      // [0, 0, 0, 1] --> [T1, T1, T1, T2] (table mapping)
      // [0, 1, 2, 0] --> [T1.0, T1.1, T1.2, T2.0] (table columns mapping)
      Byte[] tagOrder = smbJoinDesc.getTagOrder();
      Map<Byte, List<Integer>> retainList = smbJoinDesc.getRetainList();
      int totalNumberColumns = 0;
      for (Byte tag : tagOrder) {
        totalNumberColumns += retainList.get(tag).size();
      }

      byte[] columnTableMappings = new byte[totalNumberColumns];
      int[] columnNumberMappings = new int[totalNumberColumns];
      int currentColumnPosition = 0;
      for (Byte tag : tagOrder) {
        for (int pos = 0; pos < retainList.get(tag).size(); pos++) {
          columnTableMappings[currentColumnPosition] = tag;
          columnNumberMappings[currentColumnPosition] = pos;
          currentColumnPosition++;
        }
      }

      // All output columns used for bucketing/sorting of the destination table should
      // belong to the same input table
      //   insert overwrite table T3
      //   select T1.key, T2.key2, UDF(T1.value, T2.value)
      //   from T1 join T2 on T1.key = T2.key and T1.key2 = T2.key2
      // is not optimized, whereas the insert is optimized if the select list is either changed to
      // (T1.key, T1.key2, UDF(T1.value, T2.value)) or (T2.key, T2.key2, UDF(T1.value, T2.value))
      // Get the input table and make sure the keys match
      List<String> outputColumnNames = smbJoinDesc.getOutputColumnNames();
      byte tableTag = -1;
      int[] columnNumbersExprList = new int[sourceTableBucketCols.size()];
      int currentColPosition = 0;
      for (ExprNodeColumnDesc bucketCol : sourceTableBucketCols) {
        String colName = bucketCol.getColumn();
        int colNumber = outputColumnNames.indexOf(colName);
        if (colNumber < 0) {
          return false;
        }
        if (tableTag < 0) {
          tableTag = columnTableMappings[colNumber];
        } else if (tableTag != columnTableMappings[colNumber]) {
          return false;
        }
        columnNumbersExprList[currentColPosition++] = columnNumberMappings[colNumber];
      }

      List<ExprNodeDesc> allExprs = smbJoinDesc.getExprs().get(tableTag);
      List<ExprNodeDesc> keysSelectedTable = smbJoinDesc.getKeys().get(tableTag);
      currentColPosition = 0;
      for (ExprNodeDesc keySelectedTable : keysSelectedTable) {
        if (!(keySelectedTable instanceof ExprNodeColumnDesc)) {
          return false;
        }
        if (!allExprs.get(columnNumbersExprList[currentColPosition++]).isSame(keySelectedTable)) {
          return false;
        }
      }

      return true;
    }