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(mysql.info)InnoDB Physical record

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Physical record structure
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   * 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.

How an auto-increment column works in InnoDB
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After a database startup, when a user first does an insert to a table
`T' where an auto-increment column has been defined, and the user does
not provide an explicit value for the column, then InnoDB executes
`SELECT MAX(auto-inc-column) FROM T', and assigns that value incremented
by one to the the column and the auto-increment counter of the table.
We say that the auto-increment counter for table `T' has been
initialized.

InnoDB follows the same procedure in initializing the auto-increment
counter for a freshly created table.

Note that if the user specifies in an insert the value 0 to the
auto-increment column, then InnoDB treats the row like the value would
not have been specified.

After the auto-increment counter has been initialized, if a user inserts
a row where he explicitly specifies the column value, and the value is
bigger than the current counter value, then the counter is set to the
specified column value. If the user does not explicitly specify a
value, then InnoDB increments the counter by one and assigns its new
value to the column.

The auto-increment mechanism, when assigning values from the counter,
bypasses locking and transaction handling. Therefore you may also get
gaps in the number sequence if you roll back transactions which have
got numbers from the counter.

The behavior of auto-increment is not defined if a user gives a negative
value to the column or if the value becomes bigger than the maximum
integer that can be stored in the specified integer type.