Each index record contains a six-byte header. The header is used to link together consecutive records, and also in row-level locking.

Records in the clustered index contain fields for all user-defined columns. In addition, there is a six-byte transaction ID field and a seven-byte roll pointer field.

If no primary key was defined for a table, each clustered index record also contains a six-byte row ID field.

Each secondary index record also contains all the primary key fields defined for the clustered index key that are not in the secondary index.

A record contains a pointer to each field of the record. If the total length of the fields in a record is less than 128 bytes, the pointer is one byte; otherwise, two bytes. The array of these pointers is called the record directory. The area where these pointers point is called the data part of the record.

Internally, InnoDB stores fixed-length character columns such as CHAR(10) in a fixed-length format. InnoDB truncates trailing spaces from VARCHAR columns. Note that MySQL may internally convert CHAR columns to VARCHAR. See Section 12.1.5.1, “Silent Column Specification Changes”.

An SQL NULL value reserves one or two bytes in the record directory. Besides that, an SQL NULL value reserves zero bytes in the data part of the record if stored in a variable length column. In a fixed-length column, it reserves the fixed length of the column in the data part of the record. Reserving the fixed space for NULL values enables an update of the column from NULL to a non-NULL value to be done in place without causing fragmentation of the index page.