原因是虽然支持备份、恢复系统表，但是恢复时并不恢复统计信息。
具体可参考：https://github.com/pingcap/br/pull/1048
But this doesn’t allow restore stats – because the table IDs of the stats backed up are old table IDs, we need more efforts to rewrite them to make them take effect after the restoration.