Mysql deadlock explanation needed

deadlock, locking, mysql, sql, transactions

Solution

The first step is determining what the two queries are:

SELECT api_key, completed, `compute_units`, created, deleted, flags, `func_name`, group_id, hostname, `is_meta`, jid, label, language, `num_children`, `parent_ujid`, priority, `process_id`, restartable, status, type, uid, ujid, version, wid FROM jobs WHERE status='new' and is_meta=0 ORDER BY priority asc,jid asc FOR UPDATE

..and:

UPDATE jobs SET status='done' WHERE jid=10099

The first is a SELECT, the second is an UPDATE. But the key is the `FOR UPDATE` at the end of the SELECT, which I emphasized in bold.

The `FOR UPDATE` syntax is for a locking read - you can read the documentation about it here. The MySQL deadlock documentation suggestes using `READ COMMITTED` if you run into locking issues like these ones.

SHOW INNODB STATUS walk through

Problem

I received the following deadlock log via "SHOW INNODB STATUS". Can someone care to explain why the transaction was aborted? It seems that Transaction 2 is holding the lock, but is also stuck requesting the same lock (except for the "waiting" part), which leads to a deadlock when Transaction 1 requires it as well. ``` ===================================== 091205 6:25:01 INNODB MONITOR OUTPUT ===================================== Per second averages calculated from the last 39 seconds ---------- SEMAPHORES ---------- OS WAIT ARRAY INFO: reservation count 233826, signal count 229982 Mutex spin waits 0, rounds 1569878, OS waits 4740 RW-shared spins 517345, OS waits 227127; RW-excl spins 4390, OS waits 1945 ------------------------ LATEST DETECTED DEADLOCK ------------------------ 091205 6:19:35 *** (1) TRANSACTION: TRANSACTION 0 479286429, ACTIVE 0 sec, process no 17618, OS thread id 2963139472 fetching rows mysql tables in use 1, locked 1 LOCK WAIT 176 lock struct(s), heap size 11584 MySQL thread id 330396, query id 97467367 64-71-26-218.static.wiline.com 64.71.26.218 autotaggeruser Sorting result SELECT api_key,completed,compute_units,created,deleted,flags,func_name,group_id,hostname,is_meta,jid,label,language,num_children,parent_ujid,priority,process_id,restartable,status,type,uid,ujid,version,wid FROM jobs WHERE status='new' and is_meta=0 ORDER BY priority asc,jid asc FOR UPDATE *** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 0 page no 17549 n bits 128 index `PRIMARY` of table `takeyourorder/jobs` trx id 0 479286429 lock_mode X waiting Record lock, heap no 61 PHYSICAL RECORD: n_fields 26; compact format; info bits 0 0: len 8; hex 800000000000277c; asc '|;; 1: len 6; hex 00001c915499; asc T ;; 2: len 7; hex 00000006e21e2a; asc *;; 3: len 8; hex 8000000000000002; asc ;; 4: len 8; hex 8000000000000845; asc E;; 5: SQL NULL; 6: len 8; hex 8000000000002773; asc 's;; 7: len 1; hex 80; asc ;; 8: len 8; hex 8000000000000002; asc ;; 9: len 16; hex 636f72656f66746865627261696e2d75; asc coreofthebrain-u;; 10: len 4; hex 80000eb8; asc ;; 11: len 1; hex 01; asc ;; 12: len 30; hex 322e362e32202872656c6561736532362d6d61696e742c20417072203139; asc 2.6.2 (release26-maint, Apr 19;...(truncated); 13: len 30; hex 5f5f6d61696e5f5f2e3c6c616d6264613e206174203c737464696e3e3a31; asc __main__.<lambda> at <stdin>:1;; 14: len 5; hex 8000000001; asc ;; 15: len 0; hex ; asc ;; 16: len 4; hex 80000000; asc ;; 17: len 4; hex 80000005; asc ;; 18: len 4; hex 4b19fb58; asc K X;; 19: len 4; hex 4b19fb77; asc K w;; 20: len 1; hex 07; asc ;; 21: len 1; hex 80; asc ;; 22: len 4; hex 80000000; asc ;; 23: len 4; hex 80000000; asc ;; 24: len 1; hex 80; asc ;; 25: len 4; hex 80001415; asc ;; *** (2) TRANSACTION: TRANSACTION 0 479286425, ACTIVE 0 sec, process no 17618, OS thread id 2971134864 starting index read, thread declared inside InnoDB 500 mysql tables in use 1, locked 1 7 lock struct(s), heap size 1024, undo log entries 3 MySQL thread id 330430, query id 97467371 64-71-26-218.static.wiline.com 64.71.26.218 autotaggeruser Updating UPDATE jobs SET status='done' WHERE jid=10099 *** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 0 page no 17549 n bits 128 index `PRIMARY` of table `takeyourorder/jobs` trx id 0 479286425 lock_mode X locks rec but not gap Record lock, heap no 61 PHYSICAL RECORD: n_fields 26; compact format; info bits 0 0: len 8; hex 800000000000277c; asc '|;; 1: len 6; hex 00001c915499; asc T ;; 2: len 7; hex 00000006e21e2a; asc *;; 3: len 8; hex 8000000000000002; asc ;; 4: len 8; hex 8000000000000845; asc E;; 5: SQL NULL; 6: len 8; hex 8000000000002773; asc 's;; 7: len 1; hex 80; asc ;; 8: len 8; hex 8000000000000002; asc ;; 9: len 16; hex 636f72656f66746865627261696e2d75; asc coreofthebrain-u;; 10: len 4; hex 80000eb8; asc ;; 11: len 1; hex 01; asc ;; 12: len 30; hex 322e362e32202872656c6561736532362d6d61696e742c20417072203139; asc 2.6.2 (release26-maint, Apr 19;...(truncated); 13: len 30; hex 5f5f6d61696e5f5f2e3c6c616d6264613e206174203c737464696e3e3a31; asc __main__.<lambda> at <stdin>:1;; 14: len 5; hex 8000000001; asc ;; 15: len 0; hex ; asc ;; 16: len 4; hex 80000000; asc ;; 17: len 4; hex 80000005; asc ;; 18: len 4; hex 4b19fb58; asc K X;; 19: len 4; hex 4b19fb77; asc K w;; 20: len 1; hex 07; asc ;; 21: len 1; hex 80; asc ;; 22: len 4; hex 80000000; asc ;; 23: len 4; hex 80000000; asc ;; 24: len 1; hex 80; asc ;; 25: len 4; hex 80001415; asc ;; *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 0 page no 17548 n bits 144 index `PRIMARY` of table `takeyourorder/jobs` trx id 0 479286425 lock_mode X locks rec but not gap waiting Record lock, heap no 73 PHYSICAL RECORD: n_fields 26; compact format; info bits 0 0: len 8; hex 8000000000002773; asc 's;; 1: len 6; hex 00001c9151f5; asc Q ;; 2: len 7; hex 800000003c0110; asc < ;; 3: len 8; hex 8000000000000002; asc ;; 4: len 8; hex 800000000000083d; asc =;; 5: SQL NULL; 6: SQL NULL; 7: len 1; hex 81; asc ;; 8: len 8; hex 8000000000000002; asc ;; 9: len 16; hex 636f72656f66746865627261696e2d75; asc coreofthebrain-u;; 10: len 4; hex 80000eb8; asc ;; 11: len 1; hex 01; asc ;; 12: len 30; hex 322e362e32202872656c6561736532362d6d61696e742c20417072203139; asc 2.6.2 (release26-maint, Apr 19;...(truncated); 13: len 30; hex 5f5f6d61696e5f5f2e3c6c616d6264613e206174203c737464696e3e3a31; asc __main__.<lambda> at <stdin>:1;; 14: len 5; hex 8000000001; asc ;; 15: len 0; hex ; asc ;; 16: len 4; hex 80000000; asc ;; 17: len 4; hex 80000005; asc ;; 18: len 4; hex 4b19fb58; asc K X;; 19: SQL NULL; 20: len 1; hex 02; asc ;; 21: len 1; hex 80; asc ;; 22: len 4; hex 80000014; asc ;; 23: len 4; hex 80000000; asc ;; 24: len 1; hex 80; asc ;; 25: SQL NULL; *** WE ROLL BACK TRANSACTION (1) ```

Original source