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MySQL (IV) - transactions

2022-07-06 02:18:00 【waldosia】

MySQL（ Four ）—— Business

1． Concept of transactions

  • A transaction is a mechanism 、 A sequence of operations , Contains a set of database operation commands , And all the commands as a whole to submit or revoke the operation request to the system , That is, all the database commands are executed , Either not .

  • A transaction is an indivisible unit of work logic , When performing concurrent operations on a database system , Transactions are the smallest unit of control .

  • Transaction is suitable for the scenario of multi-user operating database system at the same time , Like a bank 、 Insurance companies and securities trading systems, etc .

  • Transactions ensure the consistency of data through the integrity of transactions .

  • Transactions can improve reliability during updating and inserting information into tables .

To put it bluntly , Transaction , It's a sequence of operations , These operations are either performed , Either not , It is an indivisible unit of work .

2． The transaction ACID characteristic

ACID, In a reliable database management system （DBMS） in , Business (transaction) It should have four characteristics : Atomicity (Atomicity) , Uniformity ( consistency)、 Isolation, （Isolation)、 persistence (Durability). These are the characteristics of a reliable database .

  • Atomicity ： A transaction is an indivisible unit of work , Either the operations in the transaction occur , Or none at all .

    ( A transaction is a complete operation , The elements of a transaction are indivisible . All elements in a transaction must be committed or rolled back as a whole . If any element in the transaction fails , Then the whole transaction will fail .)

Case study :
A to B transfer 100 Only the deduction statement was executed when the amount of RMB was RMB , Just submitted , If there is a sudden power failure ,A The account has been deducted ,B The account didn't receive the additional payment , It can cause disputes in life . In this case, the atomicity of transactions is needed to ensure that all transactions are executed , Or they don't do it .

  • Uniformity : Before and after the transaction , The database integrity constraint is not broken .

    ( When the transaction is complete , The data must be in a consistent state . Before the transaction starts , The data stored in the database is in a consistent state . In an ongoing transaction , The data may be in an inconsistent state . When the transaction completes successfully , The data must return to the known consistent state again .)

Case study :
For bank transfers , Whether the transaction succeeds or fails , You should make sure that after the transaction, the table A and B The total amount of deposit is the same as before .

  • Isolation, : In a concurrent environment , When different transactions manipulate the same data at the same time , Each transaction has its own full data space .

    ( All concurrent transactions that modify data are isolated from each other , Indicates that the transaction must be independent , It should not in any way depend on or affect other transactions . A transaction that modifies data can access that data before another transaction that uses the same data starts , Or access the data after another transaction using the same data ends . That is, when accessing the database concurrently , One user's transaction is not interfered by other transactions , The database between concurrent transactions is independent .)

// When multiple clients access the same table concurrently , The following consistency problems may occur :
(1） Dirty reading : One transaction reads uncommitted data from another transaction , And it's possible to roll back this data .
(2） It can't be read repeatedly : Two identical queries in a transaction return different data . This is caused by the commit of other transaction modifications in the system during query .
(3） Fantasy reading : A transaction modifies the data in a table , This modification involves all data rows in the table . meanwhile , Another transaction also modifies the data in this table , This modification is to insert a new row of data into the table . that , The user operating the previous transaction will find that there are no modified data rows in the table , It's like an illusion .
(4) Lost update : Two transactions read the same record at the same time ,A Change the record first ,B Also modify the record （B I do not know! A A modified ),B After submitting the data B The modification result covers A Modification results of .

// The isolation level of transactions determines the level of visibility between transactions .
MySQL Transaction supports the following four kinds of isolation , To control the changes made by the transaction , And notify the modification to other concurrent transactions :

（1）、 Uncommitted read (Read Uncommitted) :

Dirty reading allowed , That is, one transaction is allowed to see the uncommitted modifications of other transactions .

（2）、 Submit to read (Read Committed) :

A transaction is allowed to see only the changes committed by other transactions , Uncommitted changes are not visible .

（3）、 Repeatable (Repeated Read) : —mysql Default isolation level

Make sure that if you execute the same... Twice in a transaction SELECT sentence , We can get the same results , Whether or not other transactions commit these changes .

（4）、 Serial read (Serializable): — It's like a lock table

Read completely serially , Completely isolate one transaction from other transactions . Every time you read, you need to obtain a table level shared lock , Reading and writing will block each other .

mysql The default transaction level is repeatable read , and Oracle and SQL Server yes read committed .

// The scope of transaction isolation level is divided into two types :

  • Global level ： Valid for all conversations
  • Session level ： Valid only for the current session

Query global transaction isolation level :

show global variables like '%isolation%';

SELECT @@global.tx_isolation;

     
    
      
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Query session transaction isolation level :

show session variables like '%isolation%';

SELECT @@session.tx_isolation;

SELECT @@tx_isolation;

     
    
      
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Set the global transaction isolation level :

set global transaction isolation level read committed;

     
    
      
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Set session transaction isolation level :

set session transaction isolation level read committed;

     
    
      
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  • persistence (Durability)： No matter whether the system fails or not , The result of the transaction is permanent ·— Once the transaction is committed , The effect of the transaction is permanently preserved in the database

  • persistence : After the transaction is completed , Changes made by the firm to the database are persisted in the database , It will not be rolled back . No matter whether the system fails or not , The result of the transaction is permanent .

（ Once the transaction is committed , The effect of the transaction is permanently preserved in the database .）

summary : In transaction management , Atomicity is the foundation , Isolation is the means , Consistency is the purpose , Persistence is the result .

3． Transaction control statement

Transaction control statement

Transaction control statement meaning
BEGIN or START TRANSACTION Open a transaction explicitly
COMMIT or COMMIT WORK Commit transaction , And make all changes that have been made to the database permanent .
ROLLBACK or ROLLBACK WORK Rolling back ends the user's transaction , And undo all pending changes .
SAVEPOINT S1 Use SAVEPOINT Allows you to create a rollback point in a transaction , There can be more than one... In a transaction SAVBPOINT;"s1" Represents the name of the rollback point .
ROLLBACK TO [SAVEPOINT] S1 Roll back the transaction to the marked point .

Case study :

use kgc;
create table account(
id int (10) primary key not null,
name varchar (40),
money double
);

insert into account values(1,'A',1000);
insert into account values(2,'B',1000);

# Test commit transactions 
begin;
update account set money= money - 100 where name='A';
commit;
quit

mysql -uroot -p
use kgc;
select * from account;

# Test rollback transactions 
begin;
update account set money= money + 100 where name='A';
rollback;





     
    
      
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4． Use set Set control transaction 

SET AUTOCOMMIT=0;						 # Disable auto submit 
SET AUTOCOMMIT=1;						 # Turn on auto submit ,Mysql The default is 1
SHOW VARIABLES LIKE 'AUTOCOMMIT';		 # see Mysql Medium AUTOCOMMIT value 

 If Auto submit is not turned on , The name of the current session connection mysql All of the operations are treated as a transaction until you type rollback|commit; Current affairs 
 In order to be end . New before the end of the current transaction mysql Unable to read the operation result of any current session while connecting .
 If Auto submit is on ,mysql Will put each sql Statement as a transaction , And then automatically commit.
 Of course, whether it's on or off ,begin; commit or rollback; It's all independent business .

use kgc;
select * from account;
SET AUTOCOMMIT=0;
update account set money= money + 100 where name='B';
select * from account;
quit

mysql -u root -p
use kgc;
select * from account;


     
    
      
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Summary
The transaction ACID principle ： Atomicity (Atomicity)、 Uniformity ( consistency)、 Isolation, （Isolation)、 persistence (Durability).

Transaction control statement ：

Transaction control method ：

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本文为[waldosia]所创，转载请带上原文链接，感谢
https://yzsam.com/2022/02/202202140026441518.html

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