When InnoDB appears hung, I know the natural reaction is to check SHOW ENGINE INNODB STATUS.

In fact, it’s the first thing I check when InnoDB tables are involved.

However, I just want to iterate how valuable SHOW FULL PROCESSLIST and/or mysqladmin debug outputs can be even when it seems mysqld is hung on on InnoDB table.

Two recent cases I’ve encountered illustrate why.

Case #1:

MySQL appeared hung on the following simple, single-row INSERT:

---TRANSACTION 0 2035648699, ACTIVE 76629 sec, process no 9047,
OS thread id 3069426592, thread declared inside InnoDB 500
mysql tables in use 1, locked 1
...
INSERT INTO test (id, parent, text) VALUES (180370, 70122, 'test table')

At least that’s what it seemed per the INNODB STATUS, but unfortunately, there wasn’t any further information to go on.

The next time it occurred, SHOW FULL PROCESSLIST was captured at the time.

…

One common cause for deadlocks when using InnoDB tables is from the existence of foreign key constraints and the shared locks (S-lock) they acquire on referenced rows.

The reason I want to discuss them though is because they are often a bit tricky to diagnose, especially if you are only looking at the SHOW ENGINE INNODB STATUS output (which might be a bit counter-intuitive since one would expect it to contain this info).

Let me show a deadlock error to illustrate (below is from SHOW ENGINE INNODB STATUS\g):

------------------------
LATEST DETECTED DEADLOCK
------------------------
111109 20:10:03
*** (1) TRANSACTION:
TRANSACTION 65839, ACTIVE 19 sec, OS thread id 4264 starting index read
mysql tables in use 1, locked 1
LOCK WAIT 6 lock struct(s), heap size 1024, 3 row lock(s), undo log entries 1
MySQL thread id 3, query id 74 localhost 127.0.0.1 root Updating
UPDATE parent SET age=age+1 WHERE id=1
*** (1) WAITING FOR THIS LOCK TO BE …