: test.t_mrg
   Op: check
Msg_type: Error
Msg_text: Unable to open underlying table which is differently defined or of
non-MyISAM type or doesn't exist
*************************** 3. row ***************************
 Table: test.t_mrg
   Op: check
Msg_type: error
Msg_text: Corrupt

Now, we know that the problem is in t3's definition.

URL: https://mariadb.com/kb/en/merge/https://mariadb.com/kb/en/merge/