Table definition cache too small

The number of table definitions (SHOW CREATE TABLE\G) that can be stored in the table definition cache (table_definition_cache). If you have a large number of tables (> 400) in your database instance, you should consider a larger table definition cache to increase your database throughput and decrease your query latency.
The command SELECT COUNT(*) FROM information_schema.tables; shows you how many tables and thus table definitions you have. The global status Open_table_definitions is the current amount of open table definitions.

For further information consult the documentation of MariaDB and MySQL.

Good values for the table_definition_cache are between 400 and 2000. A bigger table definition cache also means that your database uses more memory (RAM)!