public void onMatch(RelOptRuleCall call) {
    AggregateRel aggRel = call.rel(0);
    UnionRel unionRel = call.rel(1);

    if (!unionRel.all) {
      // This transformation is only valid for UNION ALL.
      // Consider t1(i) with rows (5), (5) and t2(i) with
      // rows (5), (10), and the query
      // select sum(i) from (select i from t1) union (select i from t2).
      // The correct answer is 15.  If we apply the transformation,
      // we get
      // select sum(i) from
      // (select sum(i) as i from t1) union (select sum(i) as i from t2)
      // which yields 25 (incorrect).
      return;
    }

    RelOptCluster cluster = unionRel.getCluster();

    List<AggregateCall> transformedAggCalls =
        transformAggCalls(
            aggRel.getCluster().getTypeFactory(),
            aggRel.getGroupSet().cardinality(),
            aggRel.getAggCallList());
    if (transformedAggCalls == null) {
      // we've detected the presence of something like AVG,
      // which we can't handle
      return;
    }

    boolean anyTransformed = false;

    // create corresponding aggs on top of each union child
    List<RelNode> newUnionInputs = new ArrayList<RelNode>();
    for (RelNode input : unionRel.getInputs()) {
      boolean alreadyUnique = RelMdUtil.areColumnsDefinitelyUnique(input, aggRel.getGroupSet());

      if (alreadyUnique) {
        newUnionInputs.add(input);
      } else {
        anyTransformed = true;
        newUnionInputs.add(
            new AggregateRel(cluster, input, aggRel.getGroupSet(), aggRel.getAggCallList()));
      }
    }

    if (!anyTransformed) {
      // none of the children could benefit from the pushdown,
      // so bail out (preventing the infinite loop to which most
      // planners would succumb)
      return;
    }

    // create a new union whose children are the aggs created above
    UnionRel newUnionRel = new UnionRel(cluster, newUnionInputs, true);

    AggregateRel newTopAggRel =
        new AggregateRel(cluster, newUnionRel, aggRel.getGroupSet(), transformedAggCalls);

    // In case we transformed any COUNT (which is always NOT NULL)
    // to SUM (which is always NULLABLE), cast back to keep the
    // planner happy.
    RelNode castRel = RelOptUtil.createCastRel(newTopAggRel, aggRel.getRowType(), false);

    call.transformTo(castRel);
  }