When I queried information_schema.innodb_trx (introduced in 5.1 with the InnoDB Plugin) I noticed there were a few transactions in LOCK WAIT state. Example: mysql [information_schema] > select trx_id,trx_state  -> from information_schema.innodb_trx; +--------+-----------+ | trx_id | trx_state | +--------+-----------+ | 7974 | LOCK WAIT | | 7962 | RUNNING | +--------+-----------+ 2 rows in set (0.00 sec) Then I made a query to join a few information_schema and performance_schema tables to find out which query was blocking my transactions. It turned out that the blocking transaction had a trx_query=NONE. So my query was block by a transaction doing nothing. That's not really helpful. Let's try to recreate the situation and see exactly what happened. I'll use two sessions for the transactions and a third to monitor the server. The first session: mysql [test] > create table t1 (id int); Query OK, 0 rows affected (0.01 sec) mysql [test] > insert...