mysql的技术文档里面有如下文字：
If you do not define a PRIMARY KEY for your table, MySQL picks the first UNIQUE index that has only NOT NULL columns as the primary key and InnoDB uses it as the clustered index. If there is no such index in the table, InnoDB internally generates a clustered index where the rows are ordered by the row ID that InnoDB assigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted. Thus, the rows ordered by the row ID are physically in insertion order.
Innodb表中在没有默认主键的情况下会生成一个6byte空间的自动增长主键，可以用select _rowid from table来查询。当在表里设置了主键之后，_rowid就是对应主键，select _rowid from table查询的是对应的主键值