エラー発生後に、MySQLサーバー上で「 show processlist; 」プロセスを確認しました
+-----+------+-----------------+------------+---------+------+--------------+----------------------------------------------------------------------+----------+
| Id | User | Host | db | Command | Time | State | Info | Progress |
+-----+------+-----------------+------------+---------+------+--------------+----------------------------------------------------------------------+----------+
| 356 | pma | localhost:64231 | NULL | Sleep | 261 | | NULL | 0.000 |
| 357 | root | localhost:64232 | 2drills-d7 | Query | 261 | Sending data | insert into drills_ch_sentence_temp select * from drills_ch_sentence | 0.000 |
| 360 | root | localhost:64996 | 2drills-d7 | Query | 0 | init | show processlist | 0.00 |
+-----+------+-----------------+------------+---------+------+--------------+----------------------------------------------------------------------+----------+