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MySQL 5.7 Reference Manual
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5.4.6 The DDL Log

The DDL log, or metadata log, records metadata operations generated by data definition statements affecting table partitioning, such as ALTER TABLE t3 DROP PARTITION p2, where we must make certain that the partition is completely dropped and that its definition is removed from the list of partitions for table t3. MySQL uses this log to recover from a crash occurring in the middle of a partitioning metadata operation.

A record of partitioning metadata operations is written to the file ddl_log.log, in the MySQL data directory. This is a binary file; it is not intended to be human-readable, and you should not attempt to modify its contents in any way.

ddl_log.log is not created until it is actually needed for recording metadata statements, and is removed following a successful start of mysqld. Thus, it is possible for this file not to be present on a MySQL server that is functioning in a completely normal manner.

ddl_log.log can hold up to 1048573 entries, equivalent to 4 GB in size. Once this limit is exceeded, you must rename or remove the file before it is possible to execute any additional DDL statements. This is a known issue (Bug #83708).

There are no user-configurable server options or variables associated with this file.