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17.7.5.1 An InnoDB Deadlock Example

The following example illustrates how an error can occur when a lock request causes a deadlock. The example involves two clients, A and B.

InnoDB status contains details of the last deadlock. For frequent deadlocks, enable global variable innodb_print_all_deadlocks. This adds deadlock information to the error log.

Client A enables innodb_print_all_deadlocks, creates two tables, 'Animals' and 'Birds', and inserts data into each. Client A begins a transaction, and selects a row in Animals in share mode:

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mysql> SET GLOBAL innodb_print_all_deadlocks = ON; Query OK, 0 rows affected (0.00 sec) mysql> CREATE TABLE Animals (name VARCHAR(10) PRIMARY KEY, value INT) ENGINE = InnoDB; Query OK, 0 rows affected (0.01 sec) mysql> CREATE TABLE Birds (name VARCHAR(10) PRIMARY KEY, value INT) ENGINE = InnoDB; Query OK, 0 rows affected (0.01 sec) mysql> INSERT INTO Animals (name,value) VALUES ("Aardvark",10); Query OK, 1 row affected (0.00 sec) mysql> INSERT INTO Birds (name,value) VALUES ("Buzzard",20); Query OK, 1 row affected (0.00 sec) mysql> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) mysql> SELECT value FROM Animals WHERE name='Aardvark' FOR SHARE; +-------+ | value | +-------+ | 10 | +-------+ 1 row in set (0.00 sec)

Next, client B begins a transaction, and selects a row in Birds in share mode:

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mysql> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) mysql> SELECT value FROM Birds WHERE name='Buzzard' FOR SHARE; +-------+ | value | +-------+ | 20 | +-------+ 1 row in set (0.00 sec)

The Performance Schema shows the locks after the two select statements:

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mysql> SELECT ENGINE_TRANSACTION_ID as Trx_Id, -> OBJECT_NAME as `Table`, -> INDEX_NAME as `Index`, -> LOCK_DATA as Data, -> LOCK_MODE as Mode, -> LOCK_STATUS as Status, -> LOCK_TYPE as Type -> FROM performance_schema.data_locks; +-----------------+---------+---------+------------+---------------+---------+--------+ | Trx_Id | Table | Index | Data | Mode | Status | Type | +-----------------+---------+---------+------------+---------------+---------+--------+ | 421291106147544 | Animals | NULL | NULL | IS | GRANTED | TABLE | | 421291106147544 | Animals | PRIMARY | 'Aardvark' | S,REC_NOT_GAP | GRANTED | RECORD | | 421291106148352 | Birds | NULL | NULL | IS | GRANTED | TABLE | | 421291106148352 | Birds | PRIMARY | 'Buzzard' | S,REC_NOT_GAP | GRANTED | RECORD | +-----------------+---------+---------+------------+---------------+---------+--------+ 4 rows in set (0.00 sec)

Client B then updates a row in Animals:

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mysql> UPDATE Animals SET value=30 WHERE name='Aardvark';

Client B has to wait. The Performance Schema shows the wait for a lock:

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mysql> SELECT REQUESTING_ENGINE_LOCK_ID as Req_Lock_Id, -> REQUESTING_ENGINE_TRANSACTION_ID as Req_Trx_Id, -> BLOCKING_ENGINE_LOCK_ID as Blk_Lock_Id, -> BLOCKING_ENGINE_TRANSACTION_ID as Blk_Trx_Id -> FROM performance_schema.data_lock_waits; +----------------------------------------+------------+----------------------------------------+-----------------+ | Req_Lock_Id | Req_Trx_Id | Blk_Lock_Id | Blk_Trx_Id | +----------------------------------------+------------+----------------------------------------+-----------------+ | 139816129437696:27:4:2:139816016601240 | 43260 | 139816129436888:27:4:2:139816016594720 | 421291106147544 | +----------------------------------------+------------+----------------------------------------+-----------------+ 1 row in set (0.00 sec) mysql> SELECT ENGINE_LOCK_ID as Lock_Id, -> ENGINE_TRANSACTION_ID as Trx_id, -> OBJECT_NAME as `Table`, -> INDEX_NAME as `Index`, -> LOCK_DATA as Data, -> LOCK_MODE as Mode, -> LOCK_STATUS as Status, -> LOCK_TYPE as Type -> FROM performance_schema.data_locks; +----------------------------------------+-----------------+---------+---------+------------+---------------+---------+--------+ | Lock_Id | Trx_Id | Table | Index | Data | Mode | Status | Type | +----------------------------------------+-----------------+---------+---------+------------+---------------+---------+--------+ | 139816129437696:1187:139816016603896 | 43260 | Animals | NULL | NULL | IX | GRANTED | TABLE | | 139816129437696:1188:139816016603808 | 43260 | Birds | NULL | NULL | IS | GRANTED | TABLE | | 139816129437696:28:4:2:139816016600896 | 43260 | Birds | PRIMARY | 'Buzzard' | S,REC_NOT_GAP | GRANTED | RECORD | | 139816129437696:27:4:2:139816016601240 | 43260 | Animals | PRIMARY | 'Aardvark' | X,REC_NOT_GAP | WAITING | RECORD | | 139816129436888:1187:139816016597712 | 421291106147544 | Animals | NULL | NULL | IS | GRANTED | TABLE | | 139816129436888:27:4:2:139816016594720 | 421291106147544 | Animals | PRIMARY | 'Aardvark' | S,REC_NOT_GAP | GRANTED | RECORD | +----------------------------------------+-----------------+---------+---------+------------+---------------+---------+--------+ 6 rows in set (0.00 sec)

InnoDB only uses sequential transaction ids when a transaction attempts to modify the database. Thererfore, the previous read-only transaction id changes from 421291106148352 to 43260.

If client A attempts to update a row in Birds at the same time, this will lead to a deadlock:

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mysql> UPDATE Birds SET value=40 WHERE name='Buzzard'; ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction

InnoDB rolls back the transaction that caused the deadlock. The first update, from Client B, can now proceed.

The Information Schema contains the number of deadlocks:

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mysql> SELECT `count` FROM INFORMATION_SCHEMA.INNODB_METRICS WHERE NAME="lock_deadlocks"; +-------+ | count | +-------+ | 1 | +-------+ 1 row in set (0.00 sec)

The InnoDB status contains the following information about the deadlock and transactions. It also shows that the read-only transaction id 421291106147544 changes to sequential transaction id 43261.

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mysql> SHOW ENGINE INNODB STATUS; ------------------------ LATEST DETECTED DEADLOCK ------------------------ 2022-11-25 15:58:22 139815661168384 *** (1) TRANSACTION: TRANSACTION 43260, ACTIVE 186 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 4 lock struct(s), heap size 1128, 2 row lock(s) MySQL thread id 19, OS thread handle 139815619204864, query id 143 localhost u2 updating UPDATE Animals SET value=30 WHERE name='Aardvark' *** (1) HOLDS THE LOCK(S): RECORD LOCKS space id 28 page no 4 n bits 72 index PRIMARY of table `test`.`Birds` trx id 43260 lock mode S locks rec but not gap Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 7; hex 42757a7a617264; asc Buzzard;; 1: len 6; hex 00000000a8fb; asc ;; 2: len 7; hex 82000000e40110; asc ;; 3: len 4; hex 80000014; asc ;; *** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 27 page no 4 n bits 72 index PRIMARY of table `test`.`Animals` trx id 43260 lock_mode X locks rec but not gap waiting Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 8; hex 416172647661726b; asc Aardvark;; 1: len 6; hex 00000000a8f9; asc ;; 2: len 7; hex 82000000e20110; asc ;; 3: len 4; hex 8000000a; asc ;; *** (2) TRANSACTION: TRANSACTION 43261, ACTIVE 209 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 4 lock struct(s), heap size 1128, 2 row lock(s) MySQL thread id 18, OS thread handle 139815618148096, query id 146 localhost u1 updating UPDATE Birds SET value=40 WHERE name='Buzzard' *** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 27 page no 4 n bits 72 index PRIMARY of table `test`.`Animals` trx id 43261 lock mode S locks rec but not gap Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 8; hex 416172647661726b; asc Aardvark;; 1: len 6; hex 00000000a8f9; asc ;; 2: len 7; hex 82000000e20110; asc ;; 3: len 4; hex 8000000a; asc ;; *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 28 page no 4 n bits 72 index PRIMARY of table `test`.`Birds` trx id 43261 lock_mode X locks rec but not gap waiting Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 7; hex 42757a7a617264; asc Buzzard;; 1: len 6; hex 00000000a8fb; asc ;; 2: len 7; hex 82000000e40110; asc ;; 3: len 4; hex 80000014; asc ;; *** WE ROLL BACK TRANSACTION (2) ------------ TRANSACTIONS ------------ Trx id counter 43262 Purge done for trx's n:o < 43256 undo n:o < 0 state: running but idle History list length 0 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 421291106147544, not started 0 lock struct(s), heap size 1128, 0 row lock(s) ---TRANSACTION 421291106146736, not started 0 lock struct(s), heap size 1128, 0 row lock(s) ---TRANSACTION 421291106145928, not started 0 lock struct(s), heap size 1128, 0 row lock(s) ---TRANSACTION 43260, ACTIVE 219 sec 4 lock struct(s), heap size 1128, 2 row lock(s), undo log entries 1 MySQL thread id 19, OS thread handle 139815619204864, query id 143 localhost u2

The error log contains this information about transactions and locks:

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mysql> SELECT @@log_error; +---------------------+ | @@log_error | +---------------------+ | /var/log/mysqld.log | +---------------------+ 1 row in set (0.00 sec) TRANSACTION 43260, ACTIVE 186 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 4 lock struct(s), heap size 1128, 2 row lock(s) MySQL thread id 19, OS thread handle 139815619204864, query id 143 localhost u2 updating UPDATE Animals SET value=30 WHERE name='Aardvark' RECORD LOCKS space id 28 page no 4 n bits 72 index PRIMARY of table `test`.`Birds` trx id 43260 lock mode S locks rec but not gap Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 7; hex 42757a7a617264; asc Buzzard;; 1: len 6; hex 00000000a8fb; asc ;; 2: len 7; hex 82000000e40110; asc ;; 3: len 4; hex 80000014; asc ;; RECORD LOCKS space id 27 page no 4 n bits 72 index PRIMARY of table `test`.`Animals` trx id 43260 lock_mode X locks rec but not gap waiting Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 8; hex 416172647661726b; asc Aardvark;; 1: len 6; hex 00000000a8f9; asc ;; 2: len 7; hex 82000000e20110; asc ;; 3: len 4; hex 8000000a; asc ;; TRANSACTION 43261, ACTIVE 209 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 4 lock struct(s), heap size 1128, 2 row lock(s) MySQL thread id 18, OS thread handle 139815618148096, query id 146 localhost u1 updating UPDATE Birds SET value=40 WHERE name='Buzzard' RECORD LOCKS space id 27 page no 4 n bits 72 index PRIMARY of table `test`.`Animals` trx id 43261 lock mode S locks rec but not gap Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 8; hex 416172647661726b; asc Aardvark;; 1: len 6; hex 00000000a8f9; asc ;; 2: len 7; hex 82000000e20110; asc ;; 3: len 4; hex 8000000a; asc ;; RECORD LOCKS space id 28 page no 4 n bits 72 index PRIMARY of table `test`.`Birds` trx id 43261 lock_mode X locks rec but not gap waiting Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 7; hex 42757a7a617264; asc Buzzard;; 1: len 6; hex 00000000a8fb; asc ;; 2: len 7; hex 82000000e40110; asc ;; 3: len 4; hex 80000014; asc ;;