自动索引主键真的很好吗?

In some DBMS like MySQL the primary key is always indexed by default. I know indexing can make operations like selection and comparison of the indexed column much faster, but it can also slow down other operations like insertion and update. There are cases when there are few selections on the primary key of a table, in which indexing will not bring much benefit. In such cases wouldn't it better not indexing the primary key?In some DBMS like MySQL the primary key is alwa