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.
MySQL system tables still exist in MySQL 8.0 and can
be viewed by issuing a
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.
See How the Data Dictionary is Upgraded. Upgrading MySQL
system tables requires running the full MySQL upgrade procedure.
See Section 2.10.3, “What the MySQL Upgrade Process Upgrades”.
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.
--no-dd-upgrade option can be
used to prevent automatic upgrade of data dictionary tables at
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.
Data dictionary tables are protected by default but can be
accessed by compiling MySQL with debugging support (using the
CMake option) and specifying the
debug option and modifier. For
information about compiling debug builds, see
Section 18.104.22.168, “Compiling MySQL for Debugging”.
Modifying or writing to data dictionary tables directly is not recommended and may render your MySQL instance inoperable.
mysql> SET SESSION debug='+d,skip_dd_table_access_check';
Use this query to retrieve a list of data dictionary tables:
mysql> SELECT name, schema_id, hidden, type FROM mysql.tables where schema_id=1 AND hidden='System';
SHOW CREATE TABLE to view
data dictionary table definitions. For example:
mysql> SHOW CREATE TABLE mysql.catalogs\G