Every InnoDB table has a special index called the clustered index where the data for the rows is stored. If you define a PRIMARY KEY on your table, the index of the primary key is the clustered index.

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.

Accessing a row through the clustered index is fast because the row data is on the same page where the index search leads. If a table is large, the clustered index architecture often saves a disk I/O when compared to the traditional solution. (In many database systems, data storage uses a different page from the index record.)

Clustered indexes means that InnoDB will append the primary key to each one of your indexes on an InnoDB table. Good to know.