MySQL 8.0 Reference Manual  /  MySQL Data Dictionary  /  Data Dictionary Limitations

Pre-General Availability Draft: 2017-07-20

14.7 Data Dictionary Limitations

This section describes temporary limitations introduced with the MySQL data dictionary.

  • Manual creation of database directories under the data directory (for example, with mkdir) is unsupported. Manually created database directories are not recognized by the MySQL Server.

  • Moving data stored in MyISAM tables by copying and moving MyISAM data files is unsupported. Tables moved using this method are not discovered by the server.

  • Simple backup and restore of individual MyISAM tables using copied data files is unsupported.

  • TRUNCATE TABLE, which is mapped to DROP TABLE and CREATE TABLE in MySQL 8.0, is temporarily non-atomic. A server exit during a TRUNCATE TABLE operation can result in a dropped table. It may also result in orphaned foreign key entries in the InnoDB SYS_FOREIGN and SYS_FOREIGN_COLS dictionary tables if the table contains foreign key constraints.

  • DDL operations take longer due to writing to storage, undo logs, and redo logs instead of .frm files.

  • The period of time during which DDL operations are vulnerable to a server exit is longer due to writing to storage, undo logs, and redo logs instead of .frm files. The minimal risk introduced by this limitation is mostly applicable to restore operations and other operations that load numerous new tables.

  • With the removal of .isl files in MySQL 8.0, offline relocation of file-per-table tablespaces created outside of the MySQL data directory is not supported.

  • If MySQL exits in the middle of an ALTER TABLE operation on an InnoDB table, you may be left with an orphan intermediate or temporary table. Removing orphan intermediate and temporary tables is unsupported.

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