Purge Configuration operations to allow time for purge operations to catch up. The default value is 0, which means there is no maximum purge lag and no delay. The  InnoDB  transaction system maintains a list of transactions that have index records delete-marked by UPDATE  or  DELETE  operations. The length of the list is the purge lag. Prior to MySQL 8.0.14, the purge lag delay is calculated by the following formula, which results in a minimum delay of 5000 microseconds: As of MySQL 8.0.14, the purge lag delay is calculated by the following revised formula, which reduces the minimum delay to 5 microseconds. A delay of 5 microseconds is more appropriate for modern systems. The delay is calculated at the beginning of a purge batch. A typical  innodb_max_purge_lag  setting for a problematic workload might be 1000000 (1 million), assuming that transactions are small, only 100 bytes in size, and it is permissible to have 100MB of unpurged table rows. The purge lag is presented as the  History list length  value in the  TRANSACTIONS  section of  SHOW ENGINE INNODB STATUS  output. The  History list length  is typically a low value, usually less than a few thousand, but a write- heavy workload or long running transactions can cause it to increase, even for transactions that are read only. The reason that a long running transaction can cause the  History list length  to increase is that under a consistent read transaction isolation level such as  REPEATABLE READ , a transaction must return the same result as when the read view for that transaction was created. Consequently, the  InnoDB multi-version concurrency control (MVCC) system must keep a copy of the data in the undo log until all transactions that depend on that data have completed. The following are examples of long running transactions that could cause the  History list length  to increase: To prevent excessive delays in extreme situations where the purge lag becomes huge, you can limit the delay by setting the  innodb_max_purge_lag_delay  variable. The  innodb_max_purge_lag_delay variable specifies the maximum delay in microseconds for the delay imposed when the innodb_max_purge_lag  threshold is exceeded. The specified  innodb_max_purge_lag_delay  value is an upper limit on the delay period calculated by the  innodb_max_purge_lag  formula. The purge system is also responsible for truncating undo tablespaces. You can configure the innodb_purge_rseg_truncate_frequency  variable to control the frequency with which the purge system looks for undo tablespaces to truncate. For more information, see Truncating Undo Tablespaces. 3130 Purge and Undo Tablespace Truncation •Running a  SELECT  query after disabling  autocommit , and forgetting to issue an explicit  COMMIT  or ROLLBACK . •A  mysqldump  operation that uses the  --single-transaction  option while there is a significant amount of concurrent DML. (purge lag/innodb_max_purge_lag - 0.5) * 10000 (purge_lag/innodb_max_purge_lag - 0.9995) * 10000 mysql> SHOW ENGINE INNODB STATUS; ... ------------ TRANSACTIONS ------------ Trx id counter 0 290328385 Purge done for trx’s n:o < 0 290315608 undo n:o < 0 17 History list length 20