@Override
  public void onMatch(RelOptRuleCall call) {
    try {
      final Aggregate aggregate = call.rel(0);
      final Join join = call.rel(1);
      final RexBuilder rexBuilder = aggregate.getCluster().getRexBuilder();
      final RelBuilder relBuilder = call.builder();

      // If any aggregate functions do not support splitting, bail out
      // If any aggregate call has a filter, bail out
      for (AggregateCall aggregateCall : aggregate.getAggCallList()) {
        if (aggregateCall.getAggregation().unwrap(SqlSplittableAggFunction.class)
            == null) {
          return;
        }
        if (aggregateCall.filterArg >= 0) {
          return;
        }
      }

      // If it is not an inner join or a semi-join, we do not push the
      // aggregate operator
      if (join.getJoinType() != JoinRelType.INNER) {
        return;
      }

      if (!allowFunctions && !aggregate.getAggCallList().isEmpty()) {
        return;
      }

      boolean groupingUnique = isGroupingUnique(join, aggregate.getGroupSet());

      if (!groupingUnique && !costBased) {
        // groupingUnique is not satisfied ; and cost based decision is disabled.
        // there is no need to check further - the transformation may not happen
        return;
      }

      // Do the columns used by the join appear in the output of the aggregate?
      final ImmutableBitSet aggregateColumns = aggregate.getGroupSet();
      final RelMetadataQuery mq = call.getMetadataQuery();
      final ImmutableBitSet keyColumns = keyColumns(aggregateColumns,
          mq.getPulledUpPredicates(join).pulledUpPredicates);
      final ImmutableBitSet joinColumns =
          RelOptUtil.InputFinder.bits(join.getCondition());
      final boolean allColumnsInAggregate =
          keyColumns.contains(joinColumns);
      final ImmutableBitSet belowAggregateColumns =
          aggregateColumns.union(joinColumns);

      // Split join condition
      final List<Integer> leftKeys = Lists.newArrayList();
      final List<Integer> rightKeys = Lists.newArrayList();
      final List<Boolean> filterNulls = Lists.newArrayList();
      RexNode nonEquiConj =
          RelOptUtil.splitJoinCondition(join.getLeft(), join.getRight(),
              join.getCondition(), leftKeys, rightKeys, filterNulls);
      // If it contains non-equi join conditions, we bail out
      if (!nonEquiConj.isAlwaysTrue()) {
        return;
      }

      // Push each aggregate function down to each side that contains all of its
      // arguments. Note that COUNT(*), because it has no arguments, can go to
      // both sides.
      final Map<Integer, Integer> map = new HashMap<>();
      final List<Side> sides = new ArrayList<>();
      int uniqueCount = 0;
      int offset = 0;
      int belowOffset = 0;
      for (int s = 0; s < 2; s++) {
        final Side side = new Side();
        final RelNode joinInput = join.getInput(s);
        int fieldCount = joinInput.getRowType().getFieldCount();
        final ImmutableBitSet fieldSet =
            ImmutableBitSet.range(offset, offset + fieldCount);
        final ImmutableBitSet belowAggregateKeyNotShifted =
            belowAggregateColumns.intersect(fieldSet);
        for (Ord<Integer> c : Ord.zip(belowAggregateKeyNotShifted)) {
          map.put(c.e, belowOffset + c.i);
        }
        final ImmutableBitSet belowAggregateKey =
            belowAggregateKeyNotShifted.shift(-offset);
        final boolean unique;
        if (!allowFunctions) {
          assert aggregate.getAggCallList().isEmpty();
          // If there are no functions, it doesn't matter as much whether we
          // aggregate the inputs before the join, because there will not be
          // any functions experiencing a cartesian product effect.
          //
          // But finding out whether the input is already unique requires a call
          // to areColumnsUnique that currently (until [CALCITE-1048] "Make
          // metadata more robust" is fixed) places a heavy load on
          // the metadata system.
          //
          // So we choose to imagine the the input is already unique, which is
          // untrue but harmless.
          //
          unique = true;
        } else {
          final Boolean unique0 =
              mq.areColumnsUnique(joinInput, belowAggregateKey, true);
          unique = unique0 != null && unique0;
        }
        if (unique) {
          ++uniqueCount;
          relBuilder.push(joinInput);
          relBuilder.project(belowAggregateKey.asList().stream().map(relBuilder::field).collect(Collectors.toList()));
          side.newInput = relBuilder.build();
        } else {
          List<AggregateCall> belowAggCalls = new ArrayList<>();
          final SqlSplittableAggFunction.Registry<AggregateCall>
              belowAggCallRegistry = registry(belowAggCalls);
          final Mappings.TargetMapping mapping =
              s == 0
                  ? Mappings.createIdentity(fieldCount)
                  : Mappings.createShiftMapping(fieldCount + offset, 0, offset,
                      fieldCount);
          for (Ord<AggregateCall> aggCall : Ord.zip(aggregate.getAggCallList())) {
            final SqlAggFunction aggregation = aggCall.e.getAggregation();
            final SqlSplittableAggFunction splitter =
                Preconditions.checkNotNull(
                    aggregation.unwrap(SqlSplittableAggFunction.class));
            final AggregateCall call1;
            if (fieldSet.contains(ImmutableBitSet.of(aggCall.e.getArgList()))) {
              call1 = splitter.split(aggCall.e, mapping);
            } else {
              call1 = splitter.other(rexBuilder.getTypeFactory(), aggCall.e);
            }
            if (call1 != null) {
              side.split.put(aggCall.i,
                  belowAggregateKey.cardinality()
                      + belowAggCallRegistry.register(call1));
            }
          }
          side.newInput = relBuilder.push(joinInput)
              .aggregate(relBuilder.groupKey(belowAggregateKey), belowAggCalls)
              .build();
        }
        offset += fieldCount;
        belowOffset += side.newInput.getRowType().getFieldCount();
        sides.add(side);
      }

      if (uniqueCount == 2) {
        // Both inputs to the join are unique. There is nothing to be gained by
        // this rule. In fact, this aggregate+join may be the result of a previous
        // invocation of this rule; if we continue we might loop forever.
        return;
      }

      // Update condition
      final Mapping mapping = (Mapping) Mappings.target(
          map::get,
          join.getRowType().getFieldCount(),
          belowOffset);
      final RexNode newCondition =
          RexUtil.apply(mapping, join.getCondition());

      // Create new join
      relBuilder.push(sides.get(0).newInput)
          .push(sides.get(1).newInput)
          .join(join.getJoinType(), newCondition);

      // Aggregate above to sum up the sub-totals
      final List<AggregateCall> newAggCalls = new ArrayList<>();
      final int groupIndicatorCount =
          aggregate.getGroupCount() + aggregate.getIndicatorCount();
      final int newLeftWidth = sides.get(0).newInput.getRowType().getFieldCount();
      final List<RexNode> projects =
          new ArrayList<>(
              rexBuilder.identityProjects(relBuilder.peek().getRowType()));
      for (Ord<AggregateCall> aggCall : Ord.zip(aggregate.getAggCallList())) {
        final SqlAggFunction aggregation = aggCall.e.getAggregation();
        final SqlSplittableAggFunction splitter =
            Preconditions.checkNotNull(
                aggregation.unwrap(SqlSplittableAggFunction.class));
        final Integer leftSubTotal = sides.get(0).split.get(aggCall.i);
        final Integer rightSubTotal = sides.get(1).split.get(aggCall.i);
        newAggCalls.add(
            splitter.topSplit(rexBuilder, registry(projects),
                groupIndicatorCount, relBuilder.peek().getRowType(), aggCall.e,
                leftSubTotal == null ? -1 : leftSubTotal,
                rightSubTotal == null ? -1 : rightSubTotal + newLeftWidth));
      }

      relBuilder.project(projects);

      boolean aggConvertedToProjects = false;
      if (allColumnsInAggregate) {
        // let's see if we can convert aggregate into projects
        List<RexNode> projects2 = new ArrayList<>();
        for (int key : Mappings.apply(mapping, aggregate.getGroupSet())) {
          projects2.add(relBuilder.field(key));
        }
        for (AggregateCall newAggCall : newAggCalls) {
          final SqlSplittableAggFunction splitter =
              newAggCall.getAggregation().unwrap(SqlSplittableAggFunction.class);
          if (splitter != null) {
            final RelDataType rowType = relBuilder.peek().getRowType();
            projects2.add(splitter.singleton(rexBuilder, rowType, newAggCall));
          }
        }
        if (projects2.size()
            == aggregate.getGroupSet().cardinality() + newAggCalls.size()) {
          // We successfully converted agg calls into projects.
          relBuilder.project(projects2);
          aggConvertedToProjects = true;
        }
      }

      if (!aggConvertedToProjects) {
        relBuilder.aggregate(
            relBuilder.groupKey(Mappings.apply(mapping, aggregate.getGroupSet()),
                Mappings.apply2(mapping, aggregate.getGroupSets())),
            newAggCalls);
      }

      RelNode r = relBuilder.build();
      boolean transform = false;
      if (uniqueBased && aggConvertedToProjects) {
        transform = groupingUnique;
      }
      if (!transform && costBased) {
        RelOptCost afterCost = mq.getCumulativeCost(r);
        RelOptCost beforeCost = mq.getCumulativeCost(aggregate);
        transform = afterCost.isLt(beforeCost);
      }
      if (transform) {
        call.transformTo(r);
      }
    } catch (Exception e) {
      if (noColsMissingStats.get() > 0) {
        LOG.warn("Missing column stats (see previous messages), skipping aggregate-join transpose in CBO");
        noColsMissingStats.set(0);
      } else {
        throw e;
      }
    }
  }
Creates an AggregateJoinTransposeRule that may push down functions.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveAggregateJoinTransposeRule.java