MySQL index, transaction and storage engine of database (2)

One 、MySQL Business

1、 Concept of transactions

• A transaction is a mechanism 、 A sequence of operations , Contains a set of database operation commands , And all the commands as a whole to submit or revoke the operation request to the system , That is, this set of database commands is either executed , Either not .
• A transaction is an indivisible unit of work logic , When performing concurrent operations on a database system , Transactions are the smallest unit of control .
• Transaction is suitable for the scenario of multi-user operating database system at the same time , Like a bank 、 Insurance companies and securities trading systems, etc .
• Transactions ensure the consistency of data through the integrity of transactions .
  Simply speaking , Transaction , It's a sequence of operations , These operations are either performed , Either not , It is an indivisible unit of work .

2、 The transaction ACID characteristic

ACID, In a reliable database management system （DBMS） in , Business (transaction) It should have four characteristics ： Atomicity （Atomicity）、 Uniformity （Consistency）、 Isolation, （Isolation）、 persistence （Durability）. These are the characteristics of a reliable database .

（ One ） Atomicity

• A transaction is an indivisible unit of work , Either the operations in the transaction occur , Or none at all .
  • A transaction is a complete operation , The elements of a transaction are indivisible .
  • All elements in a transaction must be committed or rolled back as a whole .
  • If any element in the transaction fails , Then the whole transaction will fail .
• for example ：
  • A to B transfer 100 Only the deduction statement was executed when the amount of RMB was RMB , Just submitted ;
  • If there is a sudden power failure ,A The account has been deducted ,B The account didn't receive the additional payment , It can cause disputes in life .
  • In this case, the atomicity of transactions is needed to ensure that all transactions are executed , Or they don't do it .

（ Two ） Uniformity

• Before and after the transaction , The database integrity constraint is not broken .
  • When the transaction is complete , The data must be in a consistent state .
  • Before the transaction starts , The data stored in the database is in a consistent state .
  • In an ongoing transaction , The data may be in an inconsistent state .
  • When the transaction completes successfully , The data must return to the known consistent state again .
• for example ：
  • For bank transfers , Whether the transaction succeeds or fails , You should make sure that after the transaction, the table A and B The total amount of deposit is the same as before .

（ 3、 ... and ） Isolation,

Isolation concept ：

• In a concurrent environment , When different transactions manipulate the same data at the same time , Each transaction has its own full data space .
• A transaction that modifies data can access that data before another transaction that uses the same data starts , Or access the data after another transaction using the same data ends .

Classification of interactions between transactions ：

• Dirty reading ： One transaction reads uncommitted data from another transaction , And it's possible to roll back this data .
• It can't be read repeatedly ： Two identical queries in a transaction return different data . This is caused by the commit of other transaction modifications in the system during query .
• Fantasy reading ： A transaction modifies the data in a table , This modification involves all data rows in the table . meanwhile , Another transaction also modifies the data in this table , This modification is to insert a new row of data into the table . that , The user operating the previous transaction will find that there are no modified data rows in the table , It's like an illusion .
• Lost update ： Two transactions read the same record at the same time ,A Change the record first ,B Also modify the record （B I do not know! A A modified ）,B After submitting the data B The result of the modification covers A Modification results of .

Mysql And the isolation level of things ：

• read uncommitted （ Read uncommitted data ） ： Don't solve dirty reading
• read committed（ Read submitted data ） ： Can solve dirty reading
• repeatable read（ Reread read ）： Can solve dirty reading and It can't be read repeatedly —mysql default
• serializable（ Serialization ）： Can solve Dirty reading It can't be read repeatedly and Fantasy reading — It's like a lock table
  mysql The default transaction level is repeatable read , and Oracle and SQL Server yes read committed .

（ Four ） Transaction related query commands

• Query global transaction isolation level ：
  
• Query session transaction isolation level ：

• Set the global transaction isolation level ：
  
• Set session transaction isolation level ：
  

3、 persistence

• After the transaction is completed , Changes made by the firm to the database are persisted in the database , It will not be rolled back .
  • No matter whether the system fails or not , The result of the transaction is permanent .
  • Once the transaction is committed , The effect of the transaction is permanently preserved in the database .
    summary ： In transaction management , Atomicity is the foundation , Isolation is the means , Consistency is the purpose , Persistence is the result .

4、 Transaction control statement

（ One ） Create table , And add data

（ Two ） Test commit transactions

（ 3、 ... and ） Test rollback transactions

（ Four ） Test multi point rollback

5、 Use set Set control transaction

If Auto submit is not turned on , The name of the current session connection mysql All of the operations are treated as a transaction until you type rollback|commit; The current transaction is closed . New before the end of the current transaction mysql Unable to read the operation result of any current session while connecting .
If Auto submit is on ,mysql Will put each sql Statement as a transaction , And then automatically commit.
Of course, whether it's on or off ,begin; commit|rollback; It's all independent business .