The problem seems to happen after a COMMIT processing was interrupted (e.g. certification failure). When such commit processing returns, it does not always release transactional MDL locks. So some MDL locks will remain even for the next queries in the connection.
And later some DDL processor get blocked because of the MDL lock.
The problem seems to happen after a COMMIT processing was interrupted (e.g. certification failure). When such commit processing returns, it does not always release transactional MDL locks. So some MDL locks will remain even for the next queries in the connection.
And later some DDL processor get blocked because of the MDL lock.