请问一个单表的 sum group by order by 的sql 应该怎么优化？

请问单个表的sum group by order by的sql应该如何优化？请问单个表的sum group by order by的sql应该如何优化？简介:sql如下: select user_id，sum(num) num from table其中xx = 1 group by user _ id order by num desc限制