Sql--- related transactions

The four characteristics of affairs

1. Atomicity （Atomicity）： Transactions are the smallest unit of execution , Division is not allowed . The atomicity of the transaction ensures that the action is either complete , Or it doesn't work at all ;

2. Uniformity （Consistency）： After the execution of the transaction , The database changes from one correct state to another ;

3. Isolation, （Isolation）： When accessing the database concurrently , One user's transaction is not interfered by other transactions , The database between concurrent transactions is independent ;

4. persistence （Durability）： After a transaction is committed . Its changes to the data in the database are persistent , Even if the database fails, it should not have any impact .

What are the problems of concurrent transactions

Dirty reading

When a transaction is accessing data and modifying the data , This modification has not yet been committed to the database , At this time, another transaction also accesses the data , And then I used this data . Because this data is not submitted yet , So the data read by another transaction is “ Dirty data ”, basis “ Dirty data ” The operation may not be correct .

What if A The value of a row of data updated is A value , But did not submit , At this point, the transaction B I checked the value of this line of data , The value you see is A value , Then transaction B Get it A Value to do various operations , however A Right again A Value has been modified , Lead to B Did a lot of operations with the wrong value .

Missing changes

When a transaction reads a data , Another transaction also accesses the data , After modifying the data in the first transaction , The second transaction also modifies the data . In this way, the modification result in the first transaction will be lost

Business 1 Read data from a table A=20, Business 2 Also read A=20, Business 1 modify A=A-1, Business 2 Also modify A=A-1, final result A=19, Business 1 The modification of is lost .

It can't be read repeatedly

Read the same data multiple times within a transaction . Before the end of the business , Another transaction also accesses the data . that , Between two reads in the first transaction , Due to the modification of the second transaction, the data read by the first transaction twice may be different . This happens when the data read twice in a transaction is different , So it's called unrepeatable reading .

such as A Checking balance in , The first view of account balance is 300, here B Withdraw money from the account again 300,A The second time I checked the account, it was found that 0.

Fantasy reading

Unreal reading is similar to nonrepeatable reading . It happens in a transaction （T1） Read a few lines of data , Then another concurrent transaction （T2） When some data is inserted . In the subsequent query , The first thing （T1） There will be more records that don't exist , It's like an illusion , So it's called Unreal reading .

Simply speaking , You have a business A, Send a message first SQL sentence , There is a condition , To query a batch of data , Such as SELECT \* FROM table WHERE id > 10 . so what , It came out at the beginning 10 Data . Then this time , Other business B Several pieces of data were inserted into the table , And business B Also submitted , At this time, several lines of data are added .
Then business A At this point, the second query , Once again, as like as two peas before. SELECT \* FROM table WHERE id > 10 This article SQL sentence , Several pieces of data were inserted due to other transactions , This led to its query 12 Data .
Unreal reading is similar to nonrepeatable reading , Unreal reading is adding or missing a few lines of data , Unrepeatable reading refers to the modification of a single piece of data .

What are the transaction isolation levels ?MySQL The default isolation level for is ?

SQL The standard defines four isolation levels ：

• READ-UNCOMMITTED( Read uncommitted )： Lowest isolation level , Allow read of uncommitted data changes , Can cause dirty reading 、 Phantom or unrepeatable reading .

• READ-COMMITTED( Read committed )： Allow to read data submitted by concurrent transactions , Can prevent dirty reading , But phantom or unrepeatable reads can still occur .

• REPEATABLE-READ( Repeatable )： Multiple reads of the same field are consistent , Unless the data is modified by the transaction itself , Can prevent dirty and unrepeatable read , But phantom reading can still happen .

• SERIALIZABLE( Serializable )： Highest isolation level , Completely obey ACID Isolation level . All transactions are executed one by one , In this way, there is no interference between transactions , in other words , This level prevents dirty reads 、 Unrepeatable reading and phantom reading .

  

  MySQL InnoDB Storage engine in REPEATABLE-READ（ Can be reread ） The transaction isolation level uses Next-Key Lock Lock algorithm , So we can avoid the production of unreal reading , It works with other database systems ( Such as SQL Server) Is different . So InnoDB The default isolation level supported by the storage engine is REPEATABLE-READ（ Can be reread ） The transaction isolation requirements can be fully guaranteed , That is to say SQL The standard SERIALIZABLE( Serializable ) Isolation level . But what you need to know is InnoDB The storage engine uses... By default REPEATABLE-READ（ Can be reread ） There will be no loss of performance .

  InnoDB There are three algorithms for storing engine locks ：

• Record lock： Locks on a single line record

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

• Next-key lock：record+gap Lock a range , Include the record itself Next-key lock That is, the data of this line and this piece , Are locked .

author ： Garden baby Hutu

this paper [ SQL--- Related affairs ] Included in Game programming ️ - database , A game development favorite ~

If the picture is not displayed for a long time , Please use Chrome browser .