MySQL Repeatable Read isolation level and Lost Update phenomena

问题

In High Performance Java Persistence book's 6.3.3.3 section it's written that Lost Update phenomena is possible in MySQL Repeatable Read isolation level. This is the screenshot:

Assuming the following(isolation level is REPEATABLE READ):

              tx1                     |                tx2
-----------------------------------------------------------------------------------
START TRANSACTION;                    |
SELECT * FROM test WHERE id = 1;      |
( say, DB_TRX_ID = 7 at this moment)   |
                                      |
                                      |  START TRANSACTION;
                                      |  SELECT * FROM test WHERE id = 1;
                                      |  UPDATE test SET name="x" WHERE id = 1;
                                      |  COMMIT;(say, makes DB_TRX_ID = 10)
                                      |
UPDATE test SET name="y" WHERE id = 1;|
COMMIT;

Question:

Upon tx1 commit will MVCC detect that the row version(DB_TRX_ID) is not equal to 7 anymore(instead it's 10) and perform a rollback ? Or the commit will be succeeded causing a Lost Update ?

回答1:

In Repeatable Read isolation level shouldn't MySQL MVCC prevent Lost Update using database level Pessimistic Locking resulting into transaction rollback?

According to the SQL standard, Repeatable Read should prevent:

dirty reads
non-repeatable reads

The standard says nothing about lost updates because the standard was designed when 2PL (Two-Phase Locking) was the facto Concurrency Control mechanism.

If you use 2PL, then the Repeatable Read isolation level will, indeed, prevent a Lost Update.

However, MVCC can provide Repeatable Reads via multiple versions of a tuple, but, in order to prevent Lost Updates, they also need the transaction scheduler to track tuple modifications for the records read by a certain transaction. Apparently, InnoDB does not work like that.

shouldn't MySQL MVCC prevent Lost Update using database-level Pessimistic Locking resulting into transaction rollback

Now, if you read carefully the Transactions chapter in my High-Performance Java Persistence book, you will find out that MVCC does not use any Pessimistic Locking in Repeatable Read. The only locks taken are the gap and next-key locks taken on the clustered index, but those don't prevent Lost Updates.

MySQL uses Pessimistic Locking for Serializable only, which provides a 2PL Concurrency Control model, even when using the MVCC_based InnoDB storage engine.

来源：https://stackoverflow.com/questions/53562850/mysql-repeatable-read-isolation-level-and-lost-update-phenomena

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mysql

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relational-database

repeatable-read