Characteristics and isolation level of database

About the transaction characteristics and isolation level of the database , Always confused and forgotten , I'm going to write an article today to record

One 、 The nature of transactions ：ACID

ACID:
-  Atomicity （Atomicity）： A transaction or all execution , Or none of them 
-  Uniformity （Consistency）： The integrity of the data before and after the transaction must be consistent 
-  Isolation, （Isolation）： Transactions do not interfere with each other 
-  persistence （Durability）： Once the transaction is committed , Changes to the database are permanent 

 Two 、 Transaction isolation ：

-  Dirty reading ： It refers to reading the data in another uncommitted transaction during one transaction 【 Read the data of another transaction that has not been committed 】
-  It can't be read repeatedly 【 modify 】： For a data in the database , Multiple queries within a transaction range return different data values 【 This is due to the query interval , Modified and committed by another transaction 】
-  Fantasy reading / Virtual reading 【 newly added 】： Unreal reading is a phenomenon that occurs when a transaction is not executed independently 【 Such as transaction T1 A data item is removed from all rows in a table “1” It is amended as follows “2” The operation of , When the transaction T2 A row of data items is inserted into the table , And this term is still going to be zero “1” And submit it to the database . And operating transactions T1 If the user views the data that has just been modified , You'll notice that there's still one line that hasn't changed , Actually this row is from the transaction T2 Added in , It's like hallucinating , This is where the phantom reading occurs .】
【 Both phantom and unrepeatable reads read another committed transaction （ This is where the dirty reading is different ）, The difference is that the unrepeatable read queries all have the same data item , Phantom reading, on the other hand, targets a whole set of data （ Like the number of Numbers ）】

3、 ... and 、 Database isolation level ：

-  Read uncommitted 【Read uncommitted】: The lowest level , There are no guarantees 
-  Read submitted 【Read committed】： Avoid dirty reading    （oracle Default level ）
-  Repeatable 【Repeatable read 】： Avoid dirty reading , It can't be read repeatedly    （mysql Default level ）
-  Serialization 【Serializable】： Avoid dirty reading 、 It can't be read repeatedly 、 The occurrence of unreal reading 

 Higher isolation level , The less efficient the execution