@Override
    protected ImmutableBitSet extractUsedFields(Project project) {
      return RelOptUtil.InputFinder.bits(project.getProjects(), null);
    }
Rule that matches an Project 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