MySQL - isolation level of transactions

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One 、 Isolation level definition for transactions

Transaction isolation level refers to the isolation degree of one transaction's modification of data from another parallel transaction .

Transaction isolation level	describe	Dirty reading	It can't be read repeatedly	Fantasy reading	Lock reading
Uncommitted read read uncommitted	A transaction can read uncommitted data of other transactions	√	√	√
Read committed read committed	A transaction can only read the committed data of other transactions		√	√
Repeatable repeatable read	A transaction reads the same data multiple times and the result is the same			√
Serializable serializable	All transactions are executed serially , There is no concurrent operation of transactions				√

Q： The stricter the transaction isolation of the database, the better ？
A： The more strict the transaction isolation of the database , The smaller the concurrent side effects , But the more it costs , Because the essence of transaction isolation is to serialize transactions to a certain extent , You need to decide which isolation level to use according to your specific business needs

Q：oracle Only 2 Transaction isolation levels ？
A： Read committed read committed、 Serializable serializable

Q：Mysql There are several transaction isolation levels ？
A：4 All of them

Two 、Mysql innodb Default transaction isolation level

Default ：REPEATABLE-READ

show VARIABLES like 'tx_isolation'

Q：innodb Why use RR As the default transaction isolation level ？
A： because innodb stay RR The level of reading solves the problem of unreal reading .

3、 ... and 、Oracle Default transaction isolation level

Default ：read-committed