@Override
    protected ImmutableBitSet extractUsedFields(Aggregate aggregate) {
      return HiveCalciteUtil.extractRefs(aggregate);
    }
Rule that matches an Aggregate on top of a Join and tries to transform it
into a SemiJoin swapping its inputs.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveSemiJoinRule.java