没用过 MySQL，看了下文档：
https://dev.mysql.com/doc/refman/8.0/en/multiple-column-indexes.html

>MySQL can use multiple-column indexes for queries that test all the columns in the index, or queries that test just the first column, the first two columns, the first three columns, and so on. If you specify the columns in the right order in the index definition, a single composite index can speed up several kinds of queries on the same table.

感觉和索引中列的顺序有关，比如下面的例子给了：
如果对 last_name，first_name 建立联合索引，但是仅仅查询 first_name，就不会用到这个索引。感觉说的和你一样啊，只要是最左前缀匹配就可以了，除非他建立索引的顺序不是 a, b, c 。