The structure of an event's data part depends on the event type:
In v1 and v3, the event type entirely determines the data format
In v4, interpretation of the data part depends on the event
type in conjunction with information from the format
description event. This is because v4 allows for an
extra headers field, the size of which is
defined in the format description event. In practice, the
extra headers field currently is empty.
The data part of an event consists of a fixed-size part and a
variable-size part. Either or both parts may be empty, depending
on the event type. (For example, a
consists only of the header part; the fixed and variable data
parts are both empty.)
The size of the event data part is the event size (contained in the header) minus the header size. The size of the fixed data part is a function of the event type. The size of the variable data part is the event size minus the size of the header, minus the size of the fixed data part.
The following principles hold across all events in a binary log file:
The fixed part of the event data is the same size for all events of a given type.
The variable part of the event data can differ in size among events of a given type.
For details about the fixed and variable parts of event data for different events, see Event Data for Specific Event Types.