MySQL建表時不設置主鍵

在mysql的技術文檔裏面有以下文字：

          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. 

翻譯一下就是：若是沒有主動設置主鍵，就會選一個不包含NULL的第一個惟一索引列做爲主鍵列，並把它用做一個彙集索引。若是沒有這樣的索引就會使用行號生成一個彙集索引，把它當作主鍵，這個行號6bytes，自增。能夠用select _rowid from table來查詢。