Is the transaction in mysql45 isolated or not?

Business id

InnoDB Each transaction has a unique transaction ID, called transaction id. It is at the beginning of the transaction to InnoDB Of the transaction system , It is strictly increasing in the order of application .

And each row of data has multiple versions . Every time a transaction updates data , Will generate a new data version , And the transaction id The transaction assigned to this data version ID, Write it down as row trx_id. meanwhile , Keep the old data version , And in the new data version , You can get information directly .

in other words , A row in a data table , There may be multiple versions (row), Each version has its own row trx_id.

Snapshots and views

stay MySQL in , There are two “ View ” The concept of ：

• One is view. It is a virtual table defined by a query statement , Execute the query statement at the time of the call and generate the result . The syntax for creating a view is create view … , And its query method is the same as the table .
• The other is InnoDB In the realization of MVCC Consistent read view used in , namely consistent read view, Used to support RC（Read Committed, Read the submission ） and RR（Repeatable Read, Repeatable ） Implementation of isolation level .

View has no physical structure , It is used to define during the execution of a transaction “ What data can I see ”.

At repeatable read isolation level , When the transaction starts “ Took a snapshot ”. Be careful , This snapshot is based on the entire library .

On the implementation , InnoDB An array is constructed for each transaction , Used to save the transaction startup moment , Right now “ active ” All the business of ID.“ active ” Refers to , Launched but not yet submitted .

A data version , For a transaction view , Except that your updates are always visible , There are three situations ：

1. Version not submitted , invisible ;
2. Version submitted , But it was submitted after the view was created , invisible ;
3. Version submitted , And it was submitted before the view was created , so .

Update logic

The updated data is read first and then written , And this read , Can only read the current value , be called “ The current reading ”（current read）.

therefore , In the execution of the transaction B When querying statements , Take a look at their own version number is 101, The version number of the latest data is also 101, It's your own update , You can use it directly , So the query results in k The value of is 3.

Here we mention a concept , It's called current reading . Actually , except update Out of statement ,select If the statement is locked , It's also the current reading .

therefore , If the business A Query statement select * from t where id=1 Revise it , add lock in share mode or for update, You can also read that the version number is 101 The data of , Back to k The value of is 3. The next two select sentence , That is to add the read lock respectively （S lock , Shared lock ） And write lock （X lock , Exclusive lock ）.

mysql> select k from t where id=1 lock in share mode;
mysql> select k from t where id=1 for update;