How the Data Dictionary is Upgraded MySQL system tables still exist in MySQL 8.0 and can be viewed by issuing a  SHOW TABLES  statement on the  mysql  system database. Generally, the difference between MySQL data dictionary tables and system tables is that data dictionary tables contain metadata required to execute SQL queries, whereas system tables contain auxiliary data such as time zone and help information. MySQL system tables and data dictionary tables also differ in how they are upgraded. The MySQL server manages data dictionary upgrades. SQL server. See How the Data Dictionary is Upgraded. Upgrading MySQL system tables requires running the full MySQL upgrade procedure. See Section 2.11.3, “What the MySQL Upgrade Process Upgrades” . How the Data Dictionary is Upgraded New versions of MySQL may include changes to data dictionary table definitions. Such changes are present in newly installed versions of MySQL, but when performing an in-place upgrade of MySQL binaries, changes are applied when the MySQL server is restarted using the new binaries. At startup, the data dictionary version of the server is compared to the version information stored in the data dictionary to determine if data dictionary tables should be upgraded. If an upgrade is necessary and supported, the server creates data dictionary tables with updated definitions, copies persisted metadata to the new tables, atomically replaces the old tables with the new ones, and reinitializes the data dictionary. If an upgrade is not necessary, startup continues without updating the data dictionary tables. Upgrade of data dictionary tables is an atomic operation, which means that all of the data dictionary tables are upgraded as necessary or the operation fails. If the upgrade operation fails, server startup fails with an error. In this case, the old server binaries can be used with the old data directory to start the server. When the new server binaries are used again to start the server, the data dictionary upgrade is reattempted. Generally, after data dictionary tables are successfully upgraded, it is not possible to restart the server using the old server binaries. As a result, downgrading MySQL server binaries to a previous MySQL version is not supported after data dictionary tables are upgraded. The  mysqld --no-dd-upgrade  option can be used to prevent automatic upgrade of data dictionary tables at startup. When  --no-dd-upgrade  is specified, and the server finds that the data dictionary version of the server is different from the version stored in the data dictionary, startup fails with an error stating that the data dictionary upgrade is prohibited. Viewing Data Dictionary Tables Using a Debug Build of MySQL Data dictionary tables are protected by default but can be accessed by compiling MySQL with debugging support (using the  -DWITH_DEBUG=1 CMake  option) and specifying the +d,skip_dd_table_access_check debug  option and modifier. For information about compiling debug builds, see Section 5.9.1.1, “Compiling MySQL for Debugging” . Warning Modifying or writing to data dictionary tables directly is not recommended and may render your MySQL instance inoperable. After compiling MySQL with debugging support, use this  SET  statement to make data dictionary tables visible to the  mysql  client session: mysql>  SELECT name, schema_id, hidden, type FROM mysql.tables where schema_id=1 AND hidden=’System’; mysql>  SET SESSION debug=’+d,skip_dd_table_access_check’; Use this query to retrieve a list of data dictionary tables: Use  SHOW CREATE TABLE  to view data dictionary table definitions. For example: 2990