Elaborate on MySQL mvcc multi version control

This paper mainly solves the problem :

1:MVCC The specific meaning of
2:mysql in MVCC How is it realized

MVCC Multi version control Before you say this, you need to know a few things

One : Business ID
a:mysql There will be ID( Only when the table is changed for the first time , Otherwise 0) Assigned transactions id Is the only self increasing ;
b:mysql Each record in the table will have a trx_id Hide columns , It records the corresponding transaction when the current data is modified ID( There's another. roll_pointer Hide columns It refers to when it is modified undo logging , It's the data of the last version )

Two :undo journal
a: When each record is modified, a undo journal , Logging is similar to recording data, that is, it saves the data information before being modified , Generally, only the modified columns are recorded

b: After a record is modified many times , Each version undo The log roll_pointer It will string the records modified many times into a version linked list ( Simply put, every undo The journal also has roll_pointer The column records the previous undo journal )
Here's the last picture

3、 ... and : Isolation level
sql altogether 4 Isolation levels Read uncommitted , Read submitted , Repeatable , Serialization ;
Here read unsubmitted The two isolation levels of serialization are to directly read the latest version , There is no such thing as multiple versions ;
So what is discussed here MVCC It usually refers to :

For the use of READ COMMITTED( Read submitted ) and REPEATABLE READ( Repeatable ) For isolation level transactions , You must ensure that you read the modified records of the committed transactions ; That is, if another transaction has modified the record but has not yet committed , You cannot directly read the latest version of the record . The core issue is You need to determine which version in the version chain is accessible to the current transaction

Solution :ReadView, Readable view ( Consistency view )

This ReadView It mainly includes 4 A more important content :
• m_ids : It's generating ReadView when , All transactions of active read / write transactions in the current system id list
• min_trx_id: It's generating ReadView when , The smallest of the active read / write transactions in the current system id, It's just m_ids Minimum of .
• max_trx_id : It's generating ReadView when , The transaction that the system should assign to the next transaction id value ( No m_ids Maximum of , It's the next one )
• creator_trx_id : Generate this ReadView The business of id

Find process :( Take the read committed isolation level as an example )
When accessing a record Just follow the steps below to determine whether a version of the record is visible .

**1:** Get the system information and generate the record readView

**2:** Find the latest version of this record , And get its trx_id value

**3:** Judge trx_id Whether the value is included in ReadView Of m_ids in ( Priority will be given here creator_trx_id,min_trx_id,max_trx_id To judge , See the notes below for details )
If it exists, the version data is unreadable , Find the previous version directly from the version chain and continue to judge ;
If it does not exist, it means that the version data is readable , Directly return this version data ;

// notes 
a: If the accessed version of  trx_id  Property value and  ReadView  Medium  creator_trx_id  Same value ,  It means that the current transaction is accessing its own modified records  , So the data of this version is readable , Go straight back to ;
b: If the accessed version of  trx_id  Attribute value is small  ReadView  Medium  min_trx_id  value , Indicates that the transaction generating this version generates  ReadView  Submitted , So this version can be accessed by the current transaction , Directly return the version data ;
c: If the accessed version of  trx_id  Property value is greater than or equal to  ReadView  Medium  max_trx_id  Indicates that the transaction generating this version is generated in the current event  ReadView  It's only opened after , So this version cannot be accessed by the current transaction , Continue to judge trx_id Whether it is included in m_ids in ;

Last :

For the implementation of repeatable read isolation level, you only need to control ReadView The generation time of
In the read submitted isolation level, each query will generate ReadView , The repeatable read only needs to be generated when the transaction is first read ReadView that will do