1、 Introduce

MVCC, Multi version concurrency control , Problem solved ： Judge whether the current transaction is visible to a line of records ( Can I read the latest result value just modified ), Here's a visibility algorithm , Before introducing the algorithm , Let's first add two concepts and MVCC The underlying principle of .
Two concepts ：

1. The current reading ： Read the latest version of the data , Always read the latest data , Such as the following sql sentence ：
   a、select……lock in share mode
   b、select……for uodate
   c、update……
   d、delete……
   e、insert……
2. Read the snapshot ： What is read is the record of historical version ,sql Statements such as select……

Underlying principle ：
MVCC Including three parts ：

1. The first part is hidden fields , Each row of records in the table will have several hidden fields invisible to the user ,

   1. DB_TRX_ID: The transaction that created or last modified the record id;
   2. DB_ROW_ID : Hide primary key （ If no primary key is set , And there is no unique exception in the table Generated when the field is empty and of integer type
   3. DB_ROW_PTR: rollback pointer , Point to the previous version of the record

2. The second part

   1. undolog： Rollback log , Before the modification record of the transaction , The original value of the record will be saved first and then modified , So that errors in the modification process can restore the original value or other transaction reads . When different transactions modify a record , Will cause the record to undolog Form a line table , That's the list , The head of the linked list is the latest record , And the chain tail is the earliest historical record .

3. The third part

   1. readview : The read view generated by the transaction during snapshot reading , It also has three fields ：
      1. trx_list: System active transactions id
      2. up_limit_id: The smallest transaction in the list id
      3. low_limit_id: Of the next transaction that has not been allocated by the system id

Next , We introduce the visibility algorithm rules ：

1. Compare first DB_TRX_ID<UP_LIMIT_ID, If it is less than , Then the current transaction can see DBTRX_ID Records ; If greater than or equal to , Then proceed to the next judgment
2. Then judge DB_TRX_ID>=LOW_LIMIT_ID, If greater than or equal to , Then represent DB_TRX_ID The record is in READ_VIEW It only appears after generation , Then it must not be visible to transactions ; If it is less than , Then go to the next step to judge
3. Judge DB_TRX_ID Is in an active transaction , If in , What's on behalf of me READ_VIEW Generation time , This transaction is still active , Not yet commit Modified data , The current transaction is also invisible , If not , It means that this transaction is in READ_VIEW It started before the generation commit, Then the result of the modification is visible .