## 14.1 Data Dictionary Schema

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 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`](information-schema.html) tables and [`SHOW`](show.html) statements. For an overview of the tables that comprise the data dictionary, see [Data Dictionary Tables](system-schema.html#system-schema-data-dictionary-tables).

 MySQL system tables still exist in MySQL 8.0 and can be viewed by issuing a [`SHOW TABLES`](show-tables.html) 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](data-dictionary-schema.html#data-dictionary-upgrade). Upgrading MySQL system tables requires running the full MySQL upgrade procedure. See [Section 2.11.3, “What the MySQL Upgrade Process Upgrades”](upgrading-what-is-upgraded.html).

### 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**](mysqld.html) [`--no-dd-upgrade`](server-options.html#option_mysqld_no-dd-upgrade) option can be used to prevent automatic upgrade of data dictionary tables at startup. When [`--no-dd-upgrade`](server-options.html#option_mysqld_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`](server-options.html#option_mysqld_debug) option and modifier. For information about compiling debug builds, see [Section 5.9.1.1, “Compiling MySQL for Debugging”](compiling-for-debugging.html).

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`](set.html) statement to make data dictionary tables visible to the [**mysql**](mysql.html) client session:

```
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';
```

 Use [`SHOW CREATE TABLE`](show-create-table.html) to view data dictionary table definitions. For example:

```
mysql> SHOW CREATE TABLE mysql.catalogs\G
```

