Chapter 14 MySQL Data Dictionary Table of Contents Important 14.1 Data Dictionary Schema 2989 Data dictionary tables are protected and may only be accessed in debug builds of MySQL. However, MySQL supports access to data stored in data dictionary tables through  INFORMATION_SCHEMA  tables and  SHOW  statements. For an overview of the tables that comprise the data dictionary, see Data Dictionary Tables. A data dictionary-enabled server entails some general operational differences compared to a server that does not have a data dictionary; see Section 14.7, “Data Dictionary Usage Differences” . Also, for upgrades to MySQL 8.0, the upgrade procedure differs somewhat from previous MySQL releases and requires that you verify the upgrade readiness of your installation by checking specific prerequisites. For more information, see Section 2.11, “Upgrading MySQL” , particularly Section 2.11.5, “Preparing Your Installation for Upgrade” . MySQL Server incorporates a transactional data dictionary that stores information about database objects. In previous MySQL releases, dictionary data was stored in metadata files, nontransactional tables, and storage engine-specific data dictionaries. 14.1 Data Dictionary Schema ........................................................................................................ 2989 14.2 Removal of File-based Metadata Storage ............................................................................... 2991 14.3 Transactional Storage of Dictionary Data ............................................................................... 2991 14.4 Dictionary Object Cache ........................................................................................................ 2991 14.5 INFORMATION_SCHEMA and Data Dictionary Integration ...................................................... 2992 14.6 Serialized Dictionary Information (SDI) ................................................................................... 2994 14.7 Data Dictionary Usage Differences ........................................................................................ 2995 14.8 Data Dictionary Limitations .................................................................................................... 2996 This chapter describes the main features, benefits, usage differences, and limitations of the data dictionary. For other implications of the data dictionary feature, refer to the “Data Dictionary Notes”  section in the MySQL 8.0 Release Notes. Benefits of the MySQL data dictionary include: •Atomic DDL. See Section 13.1.1, “Atomic Data Definition Statement Support” . •A simpler and improved implementation for some  INFORMATION_SCHEMA  tables. See Section 14.5, “INFORMATION_SCHEMA and Data Dictionary Integration” . •Uniform and centralized caching for dictionary objects. See Section 14.4, “Dictionary Object Cache” . •Transactional, crash-safe storage of dictionary data. See Section 14.3, “Transactional Storage of Dictionary Data” . •Simplicity of a centralized data dictionary schema that uniformly stores dictionary data. See Section 14.1, “Data Dictionary Schema” . •Removal of file-based metadata storage. See Section 14.2, “Removal of File-based Metadata Storage” .