Demonstration and solution of dirty reading, unrepeatable reading and unreal reading

• Catalog

  Dirty reading

  Question presentation

  solve

  It can't be read repeatedly

  Question presentation

  solve

  Fantasy reading

  Question presentation

  solve

  ---

• Dirty reading

• Question presentation

• First set the isolation level to read uncommitted
• Then open two windows
• Operate on a table at the same time
• Both open transactions
• Such as ： Li Si wants to borrow Zhang San 500 element
• 
• 

• --  Set the transaction isolation level to read uncommitted
  SET GLOBAL TRANSACTION ISOLATION LEVEL READ uncommitted;
  
  --  Open transaction 
  START TRANSACTION;
  
  --  Transfer accounts 
  UPDATE account SET money=money-500 WHERE name=' Zhang San ';
  UPDATE account SET money=money+500 WHERE name=' Li Si ';
  
  --  Inquire about account surface 
  SELECT * FROM account;
  
  --  Roll back 
  ROLLBACK;

• Problem analysis
• At this time, Zhang San turns to Li Si 500 了 , But not submitted
• And ask Li Si to inquire
• Li Si checked again that the money had indeed arrived
• Then Zhang San said he needed Li Si to fight 500 Your IOU
• Li Si gave him an IOU , This has already taken effect in the legal sense
• After Zhang San got the IOU , Rolled back
• After the rollback is completed, the data is restored again
• After a while , Li Si goes to get the money 1500 element , Insufficient balance can be found
• At this time, Li Si went to check the balance and returned to 1000 element
• In this way, Li Si not only didn't receive the money , I also wrote a note to Zhang San 500 Your IOU
• This is a problem caused by dirty reading ：
• Data in another uncommitted transaction was read during one transaction

• solve

• Modify the transaction isolation level to read committed
• 

• --  Set the transaction isolation level to read committed
  SET GLOBAL TRANSACTION ISOLATION LEVEL read committed;
  
  --  Open transaction 
  START TRANSACTION;
  
  --  Transfer accounts 
  UPDATE account SET money=money-500 WHERE name=' Zhang San ';
  UPDATE account SET money=money+500 WHERE name=' Li Si ';
  
  --  Inquire about account surface 
  SELECT * FROM account;
  
  --  Roll back 
  ROLLBACK;

• It can't be read repeatedly

• Question presentation

• First set the isolation level to read committed
• Then open two windows
• Operate on a table at the same time
• Both open transactions
• Such as ： Li Si wants to borrow Zhang San 500 element
• 
• Problem analysis
• For example, the boss of the company asked Xiao Wang to inquire about the revenue of the company in that year
• What Xiao Wang found in the morning was 100 ten thousand
• It's time for the meeting , Will take this 100 Wan told the boss
• But it changed again in the afternoon , Turned into 80 ten thousand
• What Xiao Wang told his boss was 100 ten thousand , The data queried by the boss is 80 ten thousand
• This shows that the data queried many times is different
• ： Read the data modified and submitted in another thing during the processing of one thing

• solve

• Modify the transaction isolation level to repeatable read
• 

• --  Set the transaction isolation level to repeatable read
  SET GLOBAL TRANSACTION ISOLATION LEVEL REPEATABLE read;
  
  --  Open transaction 
  START TRANSACTION;
  
  --  Transfer accounts 
  UPDATE account SET money=money-500 WHERE name=' Zhang San ';
  UPDATE account SET money=money+500 WHERE name=' Li Si ';
  
  --  Inquire about account surface 
  SELECT * FROM account;
  
  --  Commit transaction 
  COMMIT;

• If you want to see the modified data in window 2
• Window 2 is also required commit, Again select see

• Fantasy reading

• Question presentation

• First set the isolation level to repeatable read
• Then open two windows
• Operate on a table at the same time
• Both open transactions
• window 1 First, only add records without submitting
• And then to the window 2 Check it out
• Discovery window 2 There are no newly added records
• So in the window 2 Add a new record again
• But there is no way to add , Will be stuck
• Back to the window 1 After the commit transaction
• window 2 Immediately report the wrong , Unable to add
• Then in the window 2 Do commit transactions
• At the window 2 Query again and find that

• solve

• Set the isolation level to serializable
• After modification, the window 2 When you inquire, you will get stuck
• Then in the window 1 Do commit transactions , window 2 You can immediately query successfully
• So you don't have to be in the window 2 Stuck when adding
• There is no need to think that there is no such operation as adding