Mysql45 talks about transaction isolation: why can't I see it after you change it?

Some basic concepts of transaction can be seen from this article I wrote before ：spring Business management

One 、 Implementation of transaction isolation

stay MySQL in , In fact, each record will record a rollback operation at the same time when it is updated . The latest value on the record , By rolling back operations , Can get the value of the previous state .

Suppose a value from 1 Has been changed in order to 2、3、4, In the rollback log, there will be records like the following .

There will be different transactions started at different times read-view, There can be multiple versions of the same record in the system , It's multi version concurrency control of database （MVCC）. When a transaction wants to see where it is read-view The value of , To rollback from the current value to its read-view In order to get the value of its transaction in the version .

Through this mechanism , Just record read-view The position of , It has nothing to do with how many times this value has been modified .

The system will judge that there is no earlier rollback log in the system read-view When , No transactions need these rollback logs anymore , The rollback log will be deleted .

About why try not to use long transactions

Long transactions mean that there will be a very old transaction view in the system . Since these transactions may access any data in the database at any time , So before this transaction is committed , All possible rollback records in the database must be kept , This can lead to a lot of storage space .

In addition to the impact on rollback segments , Long transactions also take up lock resources , It could also drag down the entire warehouse , We will expand the lock later .

Two 、 How to start a transaction

MySQL There are several ways to start a transaction ：

1. Start the transaction statement explicitly , begin or start transaction. The accompanying commit statement is commit, The rollback statement is rollback.
2. set autocommit=0, This command will turn off the automatic submission of this thread . It means if you only execute one select sentence , This transaction starts , And it doesn't automatically commit . This business continues until you take the initiative to commit or rollback sentence , Or disconnect .

Some client connection frameworks will default to one after successful connection set autocommit=0 The order of . This results in the following queries being in the transaction , If it's a long connection , It leads to an unexpected long affair .

therefore , I would suggest that you always use set autocommit=1, Start the transaction through an explicit statement .

But some development students will struggle “ One more interaction ” The problem of . For a business that requires frequent use of transactions , The second way is that each transaction does not need to be actively executed once at the beginning “begin”, Reduce the number of statement interactions . If you have that concern , I suggest you use commit work and chain grammar .

stay autocommit by 1 Under the circumstances , use begin Explicitly started transactions , If you execute commit Then commit the transaction . If you execute commit work and chain, Commit the transaction and start the next transaction automatically , This also saves the need to execute again begin Statement overhead . At the same time, the benefit is to know clearly whether each statement is in a transaction from the perspective of program development .

You can information_schema Library innodb_trx Query long transactions in this table , Take the following statement , Used to find a duration greater than 60s The business of .

select * from information_schema.innodb_trx where TIME_TO_SEC(timediff(now(),trx_started))>60