[MySQL] locking mechanism in MySQL

a2m8l1r 2020-11-09 12:17:48
mysql locking mechanism mysql


One 、 classification

MySQL Different storage engines support different lock mechanisms , It is divided into watch level locks 、 Row-level locks 、 Page lock .MyISAM and MEMORY The storage engine uses table level locks (table-level locking);BDB The storage engine uses page locks (page-level locking), But it also supports table level locks ;InnoDB The storage engine supports both row level locks (row-level locking), Table level locks are also supported , But the default is row level locking

 

Table lock : Low overhead , Locked fast ; A deadlock will not occur ; Large locking size , The highest probability of lock collisions , Lowest degree of concurrency . 
Row-level locks : Spending big , Lock the slow ; A deadlock occurs ; Locking granularity minimum , The lowest probability of lock collisions , The highest degree of concurrency . 
Page lock : Cost and lock time are between table lock and row lock ; A deadlock occurs ; Lock granularity is between table lock and row lock , The concurrency is average  

Two 、 Table lock

MySQL There are two modes of table level lock : Table share read lock (Table Read Lock) and Table Write Lock (Table Write Lock)

Yes MyISAM Read operation of table , Does not block other users' read requests for the same table , But it blocks write requests to the same table ; Yes MyISAM Write operation of table , Will block other users to read and write to the same table .

MyISAM In the execution of the query statement (SELECT) front , Will automatically lock all tables involved , Is performing an update operation (UPDATE、DELETE、INSERT etc. ) front , Will automatically lock the tables involved , This process does not require user intervention , therefore , Users generally don't need to use LOCK TABLE Order to MyISAM The watch is explicitly locked .

3、 ... and 、 Row-level locks

InnoDB And MyISAM There are two big differences : One is to support affairs (TRANSACTION); The second is the use of row level lock .

Row level locks also support read locks and write locks .

1、 How to lock ?

mysql InnoDB The default data modification statement of the engine :update,delete,insert Will automatically add an exclusive lock to the data involved ,select Statement will not add any lock type by default , If an exclusive lock is added, it can be used select …for update sentence , Add shared lock to use select … lock in share mode sentence . Therefore, data lines with exclusive locks cannot be modified in other transactions , It can't pass for update and lock in share mode Query data by lock , But you can go straight through select …from… Query data , Because normal queries don't have any locking mechanism .

 2、 Realization principle

InnoDB Row locking is achieved by locking the index entries on the index , This point MySQL And Oracle Different , The latter is realized by locking the corresponding data row in the data block .InnoDB This row lock implementation feature means : Data is retrieved only through index conditions , And request sharing or exclusive lock ,InnoDB To use row level locks , otherwise ,InnoDB Table locks will be used ! 

Four 、 Lock algorithm

  • record lock: Locks for individual records .

  • gap lock: Clearance lock , Lock a range , Not including the record itself

  • next-key lock:gap lock+record lock

Default isolation level ( Repeatable ) Next , The default addition is next-key lock( To solve the problem of unreal reading ), When the index contains unique attributes ( unique index , primary key ), Will be relegated to record lock.

Under read committed isolation level , Plus record lock

1、 example 1

Now watch z, Yes a,b Two ,a It's the primary key , The whole table has only one primary key index . Now the record is as follows :(1,1)(3,1)(5,3)(7,6)(10,8)

select * from z where b=3 for update

because b No index , So it's a full scan . Because locking is achieved by locking the index , Because there is no index , All will lock the whole table , That is, watch lock

 2、 example 2

Now watch z, Yes a,b Two ,a It's the primary key index ,b Building a secondary index . Now the record is as follows :(1,1)(3,1)(5,3)(7,6)(10,8)

select * from z where a=3 for update

The lock on the primary key index is record lock, To record (3,1) Lock

select * from z where b=3 for update

Because locking is achieved by locking the index . So we need to lock the primary key index and the secondary index , The primary key index will be locked by next-key The lock degenerates into record lock, The lock on the secondary index is next-key lock, The lock range is (1,3)、3、(3,6)

5、 ... and 、select Several types of

1、 Read the snapshot

The snapshot version is read , That's the historical version . ordinary SELECT That's snapshot reading

2、 The current reading

Read the latest version .

UPDATE、DELETE、INSERT、SELECT ...  LOCK IN SHARE MODE、SELECT ... FOR UPDATE It's the current reading .

Default repeatable read isolation level , Using snapshot read

Read committed uses the current read

3、 Consistent non-locked reads

The realization principle is through MVCC Mechanism realization , If the read row is in update or delete in , Read operations don't wait on the line X The release of the lock , It's to read the snapshot data of the row .

MVCC, Multi version concurrency control technology . stay  InnoDB in , Add two hidden columns after each row of records , Record creation version number and delete version number . By version number and line lock , So as to improve the concurrent performance of database system .

Consistent non lock read can greatly improve the concurrent performance

Different transaction isolation levels , The snapshot version read is different

  • Read committed isolation level , Always read the latest snapshot version . It may lead to phantom reading

  • Repeatable read isolation level , Always read the snapshot version read for the first time since the transaction started . It can avoid the production of unreal reading

4、 Consistency lock read

Under default configuration , Use a repeatable isolation level , Read data to take a consistent non lock read .

But in some scenarios, it is necessary to lock the read operation to ensure strict data consistency , At this time, you can explicitly lock the read record :

  • select *** for update( Add... To the read record X lock )

  • Lock the index record , In this case, follow UPDATE The lock case is the same

  • select *** lock in share model( Add... To the read record S lock )

  • Assume Shared locks for records , In this way , Other transactions can only be read and cannot be modified , Until the current transaction commits

author :leon66666

Source :http://www.cnblogs.com/wangzhongqiu/


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