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Re: How does Innodb cleverly avoid read-write conflicts when implementing mvcc?
Posted by: Y M
Date: September 04, 2021 08:32PM

Several locks are introduced in MySQL, among which SELECT ... FROM is a consistent read, a snapshot of the read data and no lock is set (unless the transaction level is SERIALIZABLE) (https://dev.mysql.com/doc/refman/5.7/en/innodb-locks-set.html)

The snapshot (MVCC) is implemented in MySQL by adding a header (including transaction version and pointer) and logical visibility rules to the tuple

But we always emphasize the design principle of the visibility rule, but ignore the fact that reading and writing to the tuple Header are two mutually exclusive actions, which can only be avoided by locking.

So how does the sentence of consistent read without lock understand? Is it just that there is no lock in a broad sense? How is the atomic reading and writing of the tuple Header designed? Is there any performance overhead? Is there any information available in this regard?

----- supplementary instruction -----

When a row(tuple) is updated, the new version of the row is kept, along with the old copy/copies. Each copy has a sequence number (transaction id) with it.

The transaction ID and the pointer to the copies are stored in the row header, that is, when creating a copies, you need to modify the row header (update the transaction ID and the pointer to the copies). When accessing the row, you need to access the row header first to judge the version (location) we want to access.

Modifying the row header and visiting row header should be two mutually exclusive actions (otherwise dirty data will be read in the case of concurrent reading and writing), and what I want to know is that MySQL performs this part (row header) How is the logic of reading and writing designed? Is it a read-write lock / spin lock or is there any other clever method´╝č

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