Deadlock between Update and Insert queries

Question

I have faced a deadlock in my application between an Update and an Insert query and I am unable to understand why locks are given in a way which causes deadlock.

Environment-

• Application - Django
• Database - MySQL 5.7
• Engine - Innodb
• Isolation Level - READ COMMITTED.
• Tables (names changed for security)-
  1. M - primary key - id
  2. MSC - has a foreign-key to M.id
     • Indexes on MSC
       1. Index on M(FK)
       2. Index on S(FK)
       3. Index on C(FK)
       4. Index on unique together constraint (M, S, C)

Queries- Following two queries (Queries truncated to show only relevant columns)-

• Update-

  UPDATE `MSC` SET `m_id` = 110, `s_id` = 1234, `c_id` = '9b39cd', WHERE `MSC`.`id` = 54362

• Insert-

  INSERT INTO `MSC` (`m_id`, `s_id`, `c_id`) VALUES (110, 1235, '9b39cd')

Deadlock-

• First the update query is triggered and then insert query is triggered but the output of SHOW ENGINE INNODB STATUS\G; shows that insert query initiated earlier.
• From the output, timing of their execution seems to be in below manner causing the deadlock-
  • Insert gets the exclusive(X) lock on MSC and waiting for shared(S) lock on foreign key M.
  • Update gets the exclusive(X) lock on M and waiting for exclusive(X) lock on foreign key MSC.
• Following is the full output-

    ------------------------
    LATEST DETECTED DEADLOCK
    ------------------------
    2017-03-17 15:41:03 0x7f8039550700
    * (1) TRANSACTION:
    TRANSACTION 7784084, ACTIVE 2 sec inserting
    mysql tables in use 1, locked 1
    LOCK WAIT 11 lock struct(s), heap size 1136, 46 row lock(s), undo log entries 25
    MySQL thread id 493648, OS thread handle 140188693010176, query id 55263589 ip-10-198-7-203.ec2.internal 10.198.7.203 root update
    INSERT INTO MSC (m_id, s_id, c_id) VALUES (110, 1235, '9b39cd')
    * (1) WAITING FOR THIS LOCK TO BE GRANTED:
    RECORD LOCKS space id 1377 page no 10 n bits 152 index PRIMARY of table "db"."M" trx id 7784084 lock mode S locks rec but not gap waiting
    Record lock, heap no 67 PHYSICAL RECORD: n_fields 42; compact format; info bits 0
     0: len 4; hex 800000ac; asc     ;;
     1: len 6; hex 00000076c69f; asc    v  ;;
     2: len 7; hex 76000001cb24c5; asc v    $ ;;
     3: len 8; hex 999be72e2e07032e; asc    ..  .;;
     4: len 8; hex 999c22fa43025221; asc   " C R!;;

*** (2) TRANSACTION:
TRANSACTION 7784095, ACTIVE 0 sec starting index read
mysql tables in use 1, locked 1
6 lock struct(s), heap size 1136, 3 row lock(s), undo log entries 2
MySQL thread id 493645, OS thread handle 140188694415104, query id 55263635 ip-10-198-3-73.ec2.internal 10.198.3.73 root updating
UPDATE `MSC` SET `m_id` = 110, `s_id` = 1234, `c_id` = '9b39cd', WHERE `MSC`.`id` = 54362
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 1377 page no 10 n bits 152 index PRIMARY of table "db"."M" trx id 7784095 lock_mode X locks rec but not gap
Record lock, heap no 67 PHYSICAL RECORD: n_fields 42; compact format; info bits 0
 0: len 4; hex 800000ac; asc     ;;
 1: len 6; hex 00000076c69f; asc    v  ;;
 2: len 7; hex 76000001cb24c5; asc v    $ ;;
 3: len 8; hex 999be72e2e07032e; asc    ..  .;;
 4: len 8; hex 999c22fa43025221; asc   " C R!;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 1410 page no 261 n bits 104 index PRIMARY of table "db"."MSC" trx id 7784095 lock_mode X locks rec but not gap waiting
Record lock, heap no 16 PHYSICAL RECORD: n_fields 16; compact format; info bits 0
 0: len 4; hex 800038e2; asc   8 ;;
 1: len 6; hex 00000076c694; asc    v  ;;
 2: len 7; hex 6f0000055b2a0e; asc o   [* ;;
 3: len 8; hex 999c22fa0d08a51c; asc   "     ;;
 4: len 8; hex 999c22fa3b0dffd8; asc   " ;   ;;

*** WE ROLL BACK TRANSACTION (2)

Questions- I am unable to understand the followings- 1. Why did the update query have to wait and couldn't get the locks when insert query got one? 2. Why does the update query needs/takes exclusive(X) lock on M table.

Please share your thoughts here. Let me know if any extra info is required.

Answer 1

Does the id value of 110 exist in the M table? Also it may be useful to wrap these individual transactions in START TRANSACTION; and COMMIT; commands to ensure the insert completes before the update tries to run.

Example:

START TRANSACTION;
INSERT INTO `MSC` (`m_id`, `s_id`, `c_id`) VALUES (110, 1235, '9b39cd')
COMMIT;

START TRANSACTION;
UPDATE `MSC` SET `m_id` = 110, `s_id` = 1234, `c_id` = '9b39cd', WHERE 
`MSC`.`id` = 54362
COMMIT;