reproduce such deadlock in mysql - mysql

Question

I got information from SHOW ENGINE INNODB STATUS
*** (1) TRANSACTION:
TRANSACTION 0 2799914, ACTIVE 1 sec, process no 4106, OS thread id 139808903796480 inserting
mysql tables in use 1, locked 1
LOCK WAIT 10 lock struct(s), heap size 1216, 7 row lock(s), undo log entries 3
MySQL thread id 4284, query id 2889649 localhost 127.0.0.1 test update
INSERT INTO shipping .....
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 436366 n bits 88 index `PRIMARY` of table `testdatabase`.`order` trx id 0 2799914 lock mode S locks rec but not gap waiting
Record lock, heap no 14 PHYSICAL RECORD: n_fields 213; compact format; info bits 0
..........;
*** (2) TRANSACTION:
TRANSACTION 0 2799913, ACTIVE 1 sec, process no 4106, OS thread id 139808905824000 starting index read, thread declared inside InnoDB 500
mysql tables in use 1, locked 1
5 lock struct(s), heap size 1216, 5 row lock(s), undo log entries 4
MySQL thread id 4290, query id 2889711 localhost 127.0.0.1 test Updating
UPDATE order
........
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 0 page no 436366 n bits 88 index `PRIMARY` of table `testdatabase`.`order` trx id 0 2799913 lock_mode X locks rec but not gap
Record lock, heap no 14 PHYSICAL RECORD: n_fields 213; compact format; info bits 0
..........
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 153737 n bits 88 index `PRIMARY` of table `testdatabase`.`order` trx id 0 2799913 lock_mode X locks rec but not gap waiting
Record lock, heap no 10 PHYSICAL RECORD: n_fields 213; compact format; info bits 0
......
*** WE ROLL BACK TRANSACTION (2)
------------
TRANSACTIONS
there is a FK in ship referring to order primary key.
I think T2 has hold x-lock, why it still need wait for a x-lock.
Can some one help me to reproduce such deadlock in mysql?
Thanks.

Answer 1

I don't know your queries, but seems like you're inserting row into child table, and then doing an update of row in parent table.
If that's true, you've hit this issue in MySQL: http://bugs.mysql.com/bug.php?id=48652
If a FOREIGN KEY constraint is defined on a table, any insert, update, or delete that requires the constraint condition to be checked sets shared record-level locks on the records that it looks at to check the constraint. InnoDB also sets these locks in the case where the constraint fails.
On a single record of the first table you have:
S lock from transaction 1 set,
S lock from transaction 2 set,
X lock from transaction 1 requested, blocked by S lock from transaction 2,
X lock from transaction 2 requested, blocked by S lock from transaction 1
Possible solution is to first update parent table, then insert row into child table. Suppose we need to increment some counter upon child row insertion, then queries would be:
UPDATE <parent row> SET count = count + 1;
INSERT <child row>; /* if the INSERT fails, roll back the trx */
If you want to update parent row only after inserting child row, you can use FOR UPDATE statement to set a lock on parent row:
SELECT <parent row> FOR UPDATE;
INSERT <child row>; /* if the INSERT fails, roll back the trx */
UPDATE <parent row> SET count = count + 1;