What is history list length?

What is history list length?

In layman’s terms, to be a bit hand-wavy, the history list length is in units of changes. If you’re a Git user, you might think of it as analogous to the commit history of the database. (Note, however, the entire history isn’t retained forever as it is in Git.)

What is MVCC in MySQL?

InnoDB multiversion concurrency control (MVCC) treats secondary indexes differently than clustered indexes. Records in a clustered index are updated in-place, and their hidden system columns point undo log entries from which earlier versions of records can be reconstructed.

What is undo log?

An undo log record contains information about how to undo the latest change by a transaction to a clustered index record. If another transaction needs to see the original data as part of a consistent read operation, the unmodified data is retrieved from undo log records.

What is InnoDB purge worker?

When a transaction updates a row in an InnoDB table, InnoDB’s MVCC implementation keeps old versions of the row in the InnoDB undo log. InnoDB has purge process that is used to delete these old versions.

How do you clear a deadlock?

Recap the solution steps:

1. Check the system_health session for deadlocks.
2. Create an extended event session to capture the deadlocks.
3. Analyze the deadlock reports and graphs to figure out the problem.
4. If it is possible to make improvements or changing the queries involved in the deadlock.

What is consistent read MySQL?

A consistent read means that InnoDB uses multi-versioning to present to a query a snapshot of the database at a point in time. The query sees the changes made by transactions that committed before that point in time, and no changes made by later or uncommitted transactions.

What is Phantom read in MySQL?

The so-called phantom problem occurs within a transaction when the same query produces different sets of rows at different times. For example, if a SELECT is executed twice, but returns a row the second time that was not returned the first time, the row is a “phantom” row.

What is difference between redo and undo?

The undo function is used to reverse a mistake, such as deleting the wrong word in a sentence. The redo function restores any actions that were previously undone using an undo.

What is MySQL undo_001?

Two default undo tablespaces are created when the MySQL instance is initialized. Default undo tablespace data files are named undo_001 and undo_002 . The corresponding undo tablespace names defined in the data dictionary are innodb_undo_001 and innodb_undo_002 .

What is InnoDB engine?

InnoDB is a storage engine for the database management system MySQL and MariaDB. 5 in 2010, it replaced MyISAM as MySQL’s default table type. It provides the standard ACID-compliant transaction features, along with foreign key support (Declarative Referential Integrity).

How do I view MySQL history?

In the user’s home directory there lives a hidden file called “. mysql_history” that stores mysql commands issued by that account.