MySQL Manual
7 MySQL Table Types
7.5.11.4 Physical Record Structure
- Each index record in InnoDB contains a header of 6 bytes. The header is used to link consecutive records together, and also in the row level locking.
- Records in the clustered index contain fields for all user-defined columns. In addition, there is a 6-byte field for the transaction id and a 7-byte field for the roll pointer.
- If the user has not defined a primary key for a table, then each clustered index record contains also a 6-byte row id field.
- Each secondary index record contains also all the fields defined for the clustered index key.
- A record contains also a pointer to each field of the record. If the total length of the fields in a record is < 128 bytes, then the pointer is 1 byte, else 2 bytes.
