MySQL stores active table descriptors in a special memory buffer called the table open cache. This buffer is controlled by configuration variables table_open_cache that hold the maximum number of table descriptors that MySQL should store in the cache, and table_open_cache_instances that stores the number of the table cache instances. With default values of table_open_cache=4000 and [&]

https://www.percona.com/blog/how-triggers-may-significantly-affect-the-amount-of-memory-allocated-to-your-mysql-server/