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14.15.2.1 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 14.15.2, “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 INFORMATION_SCHEMA.INNODB_TRX.

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 INFORMATION_SCHEMA.INNODB_LOCKS.

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 INFORMATION_SCHEMA.INNODB_LOCK_WAITS.

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 14.15.2.3, “Persistence and Consistency of InnoDB Transaction and Locking Information”.

The following table shows the contents of INFORMATION_SCHEMA.PROCESSLIST 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 INFORMATION_SCHEMA.INNODB_TRX 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 INFORMATION_SCHEMA.INNODB_LOCK_WAITS 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 INFORMATION_SCHEMA.INNODB_LOCKS 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

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