The undo log in MySQL is a mechanism that records the changes made to a database by transactions. It allows for rolling back or undoing changes made by a transaction, in the event of a failure or error.
The undo log consists of a set of records, each of which contains information about a change made to a data page by a transaction. This information includes the old values of the modified columns and pointers to the previous versions of the data page.
When a transaction modifies a data page, it first creates an undo record for that modification. If the transaction needs to be rolled back, the undo log is used to restore the data page to its previous state.
The undo log is also used in other scenarios, such as when a row is deleted or when a transaction updates a row that is currently being read by another transaction.
Overall, the undo log is an important component of MySQL’s transaction processing system, providing the ability to recover from errors and maintain consistency in the database.
