What does undo log mean in mysql

Editor to share with you what undo log refers to in mysql. I hope you will get something after reading this article. Let's discuss it together.

1. Undo log is to realize the atomicity of transactions. In the innodb storage engine of mysql database, undolog implements multi-version concurrency control.

2. Before operating any data, first back up the data to a place where the data backup is called undo log.

Example

/ / the number of undo page to be cleaned for each purge operation mysql > show variables like 'innodb_purge_batch_size' +-+-+ | Variable_name | Value | +-+-+ | innodb_purge_batch_size | 300 | + -+ / / is used to control the length of history list If the length is greater than this parameter, it will "delay" the operation of DML. The default value of this parameter is 0, which means that there are no restrictions on history list. Mysql > show variables like 'innodb_max_purge_lag' +-- +-+ | Variable_name | Value | +-+-+ | innodb_max_purge_lag | 0 | +-+-+ / when innodb_max_purge_lag is greater than 0 Will delay the operation of DML, the delay algorithm is: delay= ((length (history_list)-innodb_max_purge_lag) * 10)-5 / to control the maximum number of milliseconds of delay. That is, when the delay value calculated above is greater than this parameter, set delay to innodb_max_purge_lag_delay to avoid unlimited waiting for other SQL threads due to slow purge operation. Mysql > show variables like 'innodb_max_purge_lag_delay' +-- +-+ | Variable_name | Value | +-+-+ | innodb_max_purge_lag_delay | 0 | +- -finished reading this article I believe you have a certain understanding of "what does undo log mean in mysql". If you want to know more about it, please follow the industry information channel. Thank you for your reading!