@Override
    protected ImmutableBitSet extractUsedFields(Project project) {
      return RelOptUtil.InputFinder.bits(project.getProjects(), null);
    }
SemiJoinRule that matches a Project on top of a Join with an Aggregate
as its right child.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveSemiJoinRule.java