14.16.2.1 Using InnoDB Transaction and Locking Information

Identifying Blocking Transactions

It is sometimes helpful to identify which transaction blocks another. The tables that contain information about InnoDB transactions and data locks enable you to determine which transaction is waiting for another, and which resource is being requested. (For descriptions of these tables, see Section 14.16.2, “InnoDB INFORMATION_SCHEMA Transaction and Locking Information”.)

Suppose that three sessions are running concurrently. Each session corresponds to a MySQL thread, and executes one transaction after another. Consider the state of the system when these sessions have issued the following statements, but none has yet committed its transaction:

• Session A:

  BEGIN;
  SELECT a FROM t FOR UPDATE;
  SELECT SLEEP(100);

• Session B:

  SELECT b FROM t FOR UPDATE;

• Session C:

  SELECT c FROM t FOR UPDATE;

In this scenario, use the following query to see which transactions are waiting and which transactions are blocking them:

SELECT
  r.trx_id waiting_trx_id,
  r.trx_mysql_thread_id waiting_thread,
  r.trx_query waiting_query,
  b.trx_id blocking_trx_id,
  b.trx_mysql_thread_id blocking_thread,
  b.trx_query blocking_query
FROM       information_schema.innodb_lock_waits w
INNER JOIN information_schema.innodb_trx b
  ON b.trx_id = w.blocking_trx_id
INNER JOIN information_schema.innodb_trx r
  ON r.trx_id = w.requesting_trx_id;

Or, more simply, use the sys schema innodb_lock_waits view:

SELECT
  waiting_trx_id,
  waiting_pid,
  waiting_query,
  blocking_trx_id,
  blocking_pid,
  blocking_query
FROM sys.innodb_lock_waits;

If a NULL value is reported for the blocking query, see Identifying a Blocking Query After the Issuing Session Becomes Idle.

waiting trx id	waiting thread	waiting query	blocking trx id	blocking thread	blocking query
A4	6	SELECT b FROM t FOR UPDATE	A3	5	SELECT SLEEP(100)
A5	7	SELECT c FROM t FOR UPDATE	A3	5	SELECT SLEEP(100)
A5	7	SELECT c FROM t FOR UPDATE	A4	6	SELECT b FROM t FOR UPDATE

In the preceding table, you can identify sessions by the “waiting query” or “blocking query” columns. As you can see:

• Session B (trx id A4, thread 6) and Session C (trx id A5, thread 7) are both waiting for Session A (trx id A3, thread 5).

• Session C is waiting for Session B as well as Session A.

You can see the underlying data in the tables INNODB_TRX, INNODB_LOCKS, and INNODB_LOCK_WAITS.

The following table shows some sample contents of the Information Schema INNODB_TRX table.

trx id	trx state	trx started	trx requested lock id	trx wait started	trx weight	trx mysql thread id	trx query
A3	RUN­NING	2008-01-15 16:44:54	NULL	NULL	2	5	SELECT SLEEP(100)
A4	LOCK WAIT	2008-01-15 16:45:09	A4:1:3:2	2008-01-15 16:45:09	2	6	SELECT b FROM t FOR UPDATE
A5	LOCK WAIT	2008-01-15 16:45:14	A5:1:3:2	2008-01-15 16:45:14	2	7	SELECT c FROM t FOR UPDATE

The following table shows some sample contents of the Information Schema INNODB_LOCKS table.

lock id	lock trx id	lock mode	lock type	lock table	lock index	lock data
A3:1:3:2	A3	X	RECORD	test.t	PRIMARY	0x0200
A4:1:3:2	A4	X	RECORD	test.t	PRIMARY	0x0200
A5:1:3:2	A5	X	RECORD	test.t	PRIMARY	0x0200

The following table shows some sample contents of the Information Schema INNODB_LOCK_WAITS table.

requesting trx id	requested lock id	blocking trx id	blocking lock id
A4	A4:1:3:2	A3	A3:1:3:2
A5	A5:1:3:2	A3	A3:1:3:2
A5	A5:1:3:2	A4	A4:1:3:2

Identifying a Blocking Query After the Issuing Session Becomes Idle

When identifying blocking transactions, a NULL value is reported for the blocking query if the session that issued the query has become idle. In this case, use the following steps to determine the blocking query:

1. Identify the processlist ID of the blocking transaction. In the sys.innodb_lock_waits table, the processlist ID of the blocking transaction is the blocking_pid value.

2. Using the blocking_pid, query the MySQL Performance Schema threads table to determine the THREAD_ID of the blocking transaction. For example, if the blocking_pid is 6, issue this query:

   SELECT THREAD_ID FROM performance_schema.threads WHERE PROCESSLIST_ID = 6;

3. Using the THREAD_ID, query the Performance Schema events_statements_current table to determine the last query executed by the thread. For example, if the THREAD_ID is 28, issue this query:

   SELECT THREAD_ID, SQL_TEXT FROM performance_schema.events_statements_current
   WHERE THREAD_ID = 28\G

4. If the last query executed by the thread is not enough information to determine why a lock is held, you can query the Performance Schema events_statements_history table to view the last 10 statements executed by the thread.

   SELECT THREAD_ID, SQL_TEXT FROM performance_schema.events_statements_history
   WHERE THREAD_ID = 28 ORDER BY EVENT_ID;

Correlating InnoDB Transactions with MySQL Sessions

Sometimes it is useful to correlate internal InnoDB locking information with the session-level information maintained by MySQL. For example, you might like to know, for a given InnoDB transaction ID, the corresponding MySQL session ID and name of the session that may be holding a lock, and thus blocking other transactions.

The following output from the INFORMATION_SCHEMA tables is taken from a somewhat loaded system. As can be seen, there are several transactions running.

The following INNODB_LOCKS and INNODB_LOCK_WAITS tables show that:

• Transaction 77F (executing an INSERT) is waiting for transactions 77E, 77D, and 77B to commit.

• Transaction 77E (executing an INSERT) is waiting for transactions 77D and 77B to commit.

• Transaction 77D (executing an INSERT) is waiting for transaction 77B to commit.

• Transaction 77B (executing an INSERT) is waiting for transaction 77A to commit.

• Transaction 77A is running, currently executing SELECT.

• Transaction E56 (executing an INSERT) is waiting for transaction E55 to commit.

• Transaction E55 (executing an INSERT) is waiting for transaction 19C to commit.

• Transaction 19C is running, currently executing an INSERT.

Note

There may be inconsistencies between queries shown in the INFORMATION_SCHEMA PROCESSLIST and INNODB_TRX tables. For an explanation, see Section 14.16.2.3, “Persistence and Consistency of InnoDB Transaction and Locking Information”.

The following table shows the contents of the Information Schema PROCESSLIST table for a system running a heavy workload.

ID	USER	HOST	DB	COMMAND	TIME	STATE	INFO
384	root	localhost	test	Query	10	update	INSERT INTO t2 VALUES …
257	root	localhost	test	Query	3	update	INSERT INTO t2 VALUES …
130	root	localhost	test	Query	0	update	INSERT INTO t2 VALUES …
61	root	localhost	test	Query	1	update	INSERT INTO t2 VALUES …
8	root	localhost	test	Query	1	update	INSERT INTO t2 VALUES …
4	root	localhost	test	Query	0	preparing	SELECT * FROM PROCESSLIST
2	root	localhost	test	Sleep	566		NULL

The following table shows the contents of the Information Schema INNODB_TRX table for a system running a heavy workload.

trx id	trx state	trx started	trx requested lock id	trx wait started	trx weight	trx mysql thread id	trx query
77F	LOCK WAIT	2008-01-15 13:10:16	77F	2008-01-15 13:10:16	1	876	INSERT INTO t09 (D, B, C) VALUES …
77E	LOCK WAIT	2008-01-15 13:10:16	77E	2008-01-15 13:10:16	1	875	INSERT INTO t09 (D, B, C) VALUES …
77D	LOCK WAIT	2008-01-15 13:10:16	77D	2008-01-15 13:10:16	1	874	INSERT INTO t09 (D, B, C) VALUES …
77B	LOCK WAIT	2008-01-15 13:10:16	77B:733:12:1	2008-01-15 13:10:16	4	873	INSERT INTO t09 (D, B, C) VALUES …
77A	RUN­NING	2008-01-15 13:10:16	NULL	NULL	4	872	SELECT b, c FROM t09 WHERE …
E56	LOCK WAIT	2008-01-15 13:10:06	E56:743:6:2	2008-01-15 13:10:06	5	384	INSERT INTO t2 VALUES …
E55	LOCK WAIT	2008-01-15 13:10:06	E55:743:38:2	2008-01-15 13:10:13	965	257	INSERT INTO t2 VALUES …
19C	RUN­NING	2008-01-15 13:09:10	NULL	NULL	2900	130	INSERT INTO t2 VALUES …
E15	RUN­NING	2008-01-15 13:08:59	NULL	NULL	5395	61	INSERT INTO t2 VALUES …
51D	RUN­NING	2008-01-15 13:08:47	NULL	NULL	9807	8	INSERT INTO t2 VALUES …

The following table shows the contents of the Information Schema INNODB_LOCK_WAITS table for a system running a heavy workload.

requesting trx id	requested lock id	blocking trx id	blocking lock id
77F	77F:806	77E	77E:806
77F	77F:806	77D	77D:806
77F	77F:806	77B	77B:806
77E	77E:806	77D	77D:806
77E	77E:806	77B	77B:806
77D	77D:806	77B	77B:806
77B	77B:733:12:1	77A	77A:733:12:1
E56	E56:743:6:2	E55	E55:743:6:2
E55	E55:743:38:2	19C	19C:743:38:2

The following table shows the contents of the Information Schema INNODB_LOCKS table for a system running a heavy workload.

lock id	lock trx id	lock mode	lock type	lock table	lock index	lock data
77F:806	77F	AUTO_INC	TABLE	test.t09	NULL	NULL
77E:806	77E	AUTO_INC	TABLE	test.t09	NULL	NULL
77D:806	77D	AUTO_INC	TABLE	test.t09	NULL	NULL
77B:806	77B	AUTO_INC	TABLE	test.t09	NULL	NULL
77B:733:12:1	77B	X	RECORD	test.t09	PRIMARY	supremum pseudo-record
77A:733:12:1	77A	X	RECORD	test.t09	PRIMARY	supremum pseudo-record
E56:743:6:2	E56	S	RECORD	test.t2	PRIMARY	0, 0
E55:743:6:2	E55	X	RECORD	test.t2	PRIMARY	0, 0
E55:743:38:2	E55	S	RECORD	test.t2	PRIMARY	1922, 1922
19C:743:38:2	19C	X	RECORD	test.t2	PRIMARY	1922, 1922