MySQL transaction mvcc principle

Let's discuss in Mysql RR and RC Under isolation level MVCC Multi version concurrency control , stay Mysql in MVCC The mechanism is to use undo log Version chain mechanism implementation ReadView Mechanism .

ReadView

ReadView What is it? , Simply speaking ,ReadView When starting a transaction , It will generate a ReadView Used to implement MVCC Mechanism . Here is ReadView Some key things contained in

m_ids: This means at this time mysql Which transactions in the transaction have not been committed
min_trx_id: Minimum business Id, Namely m_ids The smallest transaction in Id
max_trx_id: The biggest business Id, Namely Mysql The next transaction to generate Id, Is the biggest thing Id
creator_trx_id: Current affairs Id

stay Mysql There are two hidden fields behind each data row in ,trx_id and roll_pointer

trx_id: Current data update transactions Id
roll_pointer: Point to next undo log Version chain , Used to find undo log Data in the version chain

RC ISOLATION Realization principle

Read submitted ： Transactions can only read committed data ; stay Mysql in , Each transaction query generates a new ReadView To achieve RC.

MVCC Realization RC Isolation level ： Each query will generate a new Readview, Let's look at the following example

1、 Here is a piece of data in the database ：

2、 If A The transaction queries the submitted data ,trx_id = 1, At this time, I will make a judgment , Just to judge trx_id Less than min_trx_id, So it proves that this data dates back to A The transaction has been committed before it starts , So you can query this data .

3、 If the transaction B(id = 25) Modified this line of data , And submitted , So this data is trx_id Will be changed to trx_id = 25, meanwhile roll_pointer Will point to trx_id = 10 That piece of data , as follows undo log Version chain . At this point, the transaction A Launch the query again , Will generate a new ReadView, So at this time trx_id stay min_trx_id and ma_trx_id Within limits , Not at the same time m_ids In the list , So it proves that this transaction change has been committed , stay RC It can be read under the isolation level .

4、 When a transaction A After modifying this data , The trx_id Changed to 20, here trx_id Although in m_ids In the list , At the same time, the current data trx_id The value of and ReadView in creator_id The value of is the same , This shows that this data is a transaction A Altered , So at this time A Transactions can be queried .

RR ISOLATION Realization principle

Repeatable ： Business A Multiple queries , The result is the same ;Mysql By generating a fixed ReadView Realization RR Isolation level .

MVCC Realization RR Isolation level ： Each query will be used when the transaction is opened ReadView, Let's look at the following example

1、 Here is a piece of data in the database ：

2、 If the transaction A(id = 20) And is a matter B(id = 25) Open the transaction at the same time , Well, at this time ReadView yes ：

3、 If the transaction A(id = 20) Open query , Find current data trx_id = 10, Less than min_trx_id, So it means that this data is in the transaction A Submitted before opening , So this data can be queried .

4、 If the transaction B(id = 25) Changed data , So at this point trx_id = 25, By looking at A Of ReadView You can see ,trx_id = 25 stay m_ids In the list , So it explains the transaction B And business A Is a transaction started at the same time , So in RR It cannot be queried under the isolation level .

5、 If the transaction A(id = 20) Changed this data again , So at this time trx_id = 20, By looking at A Of ReadView You can see ,trx_id = 20 stay m_ids In the list , however trx_id = 20 And A Of ReadView in creator_trx_id The value of is the same , So this data is A The transaction itself changes , So it can be queried .

6、 If there is a new transaction at this time C(id = 30) Changed this data , So at this time trx_id = 30, By inquiring ReadView You can see ,trx_id = 30 No longer m_ids In the list , however trx_id Greater than max_trx_id, So it means that this data is in A Started after the transaction is started , Therefore, it is impossible to query .