The document discusses the transactional data dictionary introduced in MySQL 8.0. It describes how previously metadata was stored inconsistently across files and non-transactional tables, causing issues. MySQL 8.0 stores all metadata transactionally in InnoDB tables, providing reliability, atomic DDL, and improved INFORMATION_SCHEMA performance of up to 100x. The new approach solves longstanding problems and enables features like automated data dictionary upgrades and disaster recovery using serialized dictionary information.