Deeply understand the characteristics of database transaction isolation

Many students don't know much about database transaction isolation , After reading it for a long time, I seem to understand it and I don't seem to understand it , Let me talk about my understanding ;
Here is a key point , Do it , Open it manually sql Command window , Knock the command by hand , A certain , A certain .

First , What is business ;
One sql Is a statement a transaction ？
Only the display is written begin ,rollback,commit; Is it a business ？

One sql Statement is a transaction , Database default begin and commit;
Show written , That sentence is even more business , So when reading books, as long as the textbook mentions affairs , You have to understand A statement is also a transaction .
Connect ：https://zhuanlan.zhihu.com/p/67808523

Log in to the database first postgres perhaps mysql/oracle

Through the command ：

show default_transaction;

Check the default isolation level of the database .

** Serialization (Serializable,SQLite The default mode ）：** The highest level of isolation . Two simultaneous transactions 100% Isolation , Each transaction has its own " The world ", Serial execution .
** Repeatable （Repeatable read,MySQL The default mode ）：** If a transaction executes successfully and new data is added ( Transaction submission ), This data is visible to other ongoing transactions . But if the transaction succeeds in modifying a piece of data , The modification result is not visible to the running transaction . therefore , Transactions only break through isolation in terms of new data , Still isolate existing data .
** Read committed （Read committed,Oracle、PostgreSQL、SQL Server The default mode ）：** Repeatable + New isolation breakthrough . If the transaction A Read the data D, Then the data D To be transacted B modify （ Or delete ） And submit , Business A Read data again D Changes in data （ Or delete ） Is visible . This is called non repeatable （non-repeatable read）.
** Read uncommitted （Read uncommitted）：** The lowest level of isolation , Yes, read committed + New isolation breakthrough . If the transaction A Read the data D, Then the data D To be transacted B modify （ But did not submit , Business B Still running ）, Business A Read data again D when , Data modification is visible . If the transaction B Roll back , The transaction A Second read data D It's meaningless , Because that's business B Changes made that never happened （ It has been rolled back ）. This is called dirty reading （dirty read）.

Serialization (Serializable,SQLite The default mode ）
And then by order ：

show transaction_isolation;

View the current isolation level .

Let's first look at the highest isolation level ：serializable

begin;
set transaction isolation level serializable;
....
commit;

The isolation level set above is serializable, Before setting , Be sure to add. begin, It means that your current business is serializable Isolation ,commit after , If the next transaction is not set , Is the default isolation level of your database .

What does this isolation level mean ？
You open another window , You don't need to show the addition begin commit, Just add it in a mess , Delete , Modifying data , While changing, you set it on the other side serializable Query in the window of , Have you found , You set up. serializable The data in this window seems to live in a copy of the moment you just entered , Nothing has changed . This is the highest specification isolation .

To sum up ：serializable Under isolation level , you are here serializable All operations in the transaction are isolated in the state of the table before you enter the transaction （ It's like giving you copy Took a snapshot , All your operations are on this copy ）, You can't see what others have changed .

Repeatable （Repeatable read,MySQL The default mode ）
In this state , You can see the submissions added by others . You can't see what others have modified and submitted .
however ,PostgreSQL Achieve more stringent , It is not only required to be repeatable , Unreal reading is not allowed yet . So it seems to be similar to serializable It's the same .

Read committed （Read committed
You can see what others have submitted ..

Read uncommitted （Read uncommitted
You can see what others haven't submitted .

in fact , stay postgresql in , Read uncommitted （read uncommitted） And read submit （read committed） The implementation of is consistent , So in essence PG Only three isolation levels are achieved .

Reference resources ：https://www.xluke.info/article/pg-isolation