Wikimedia Bugzilla is closed!

wb_changes should use a robust serialization mechanism. Serialized PHP objects often become incompatible due to changes the the class definition. Serializing nested array structures instead (as JSON or using php serialization) would avoid this.

wb_changes is accessed by client side code which may be running a different Wikibase version than the repository. The client version may be older or new as the repo version, depending on how and where it is dunning. Also, old entries in wb_changes may easily have been created by an old version of the repository.