Tell you about mvcc

Long ago , An interviewer asked , You know,  mysql  Transaction isolation level ,“ forehead  O__O …, Don't know much about it ”, After that, I went to the Internet to find relevant articles , Found this

MySQL  Description of four transaction isolation levels

,  The article is very well written , After reading this, you can understand the transaction isolation levels , However, after reading this, if you think more about it, you will still find problems , How is such a magical level of transaction isolation achieved

among  innodb  The transaction isolation of the uses the  mvcc,

Multiversion concurrency control

,  Multi version concurrency control , Let's not talk about what this is , Consider if pure speculation , How should this transaction isolation level be implemented , Stupid I thought , You can copy a table at the beginning of a transaction , This can support  RR  Level ,RC  Level is not supported , And if it's a very big watch , It is not feasible to think about it

He said with a shy face that although this was not feasible , But the idea is right , A series of measures should be taken to implement it （ Fat intestines ） The changes to the , First of all, according to my understanding , In fact, copying a table becomes copying a record , But if there are multiple transactions , You have to copy it many times , This problem can be explained with the help of version management system , Version management system in use ,git  And so on , Very primitive may be after developing a function , Just make a compressed package and name it with time and other information , Then, if you want to retrieve this later, you can directly use the compressed package , Then there was.  svn,git  Centralized and distributed version management system , One of its characteristics is that the granularity can be controlled to the file and code line level , Corresponding to our  mysql  Can transactions also be refined from the table level expected at the beginning to the row level , Maybe many people have known it before , In addition to our user-defined fields, a row of records in the database , There are some additional fields , To source code

data0type.h

Fish in

/* Precise data types for system columns and the length of those columns;
NOTE: the values must run from 0 up in the order given! All codes must
be less than 256 */
#define DATA_ROW_ID 0 /* row id: a 48-bit integer */
#define DATA_ROW_ID_LEN 6 /* stored length for row id */

/** Transaction id: 6 bytes */
constexpr size_t DATA_TRX_ID = 1;

/** Transaction ID type size in bytes. */
constexpr size_t DATA_TRX_ID_LEN = 6;

/** Rollback data pointer: 7 bytes */
constexpr size_t DATA_ROLL_PTR = 2;

/** Rollback data pointer type size in bytes. */
constexpr size_t DATA_ROLL_PTR_LEN = 7;


One is  

DATA_ROW_ID

, This is to generate a hidden when the data does not specify a primary key , If the user specifies a primary key, it is the primary key

One is  

DATA_TRX_ID

, This represents the transaction of this record  ID

The other one is  

DATA_ROLL_PTR

  Pointer to rollback segment

The rollback segment pointed to is actually what we often say  undo log, The specific structure is a linked list , stay  mvcc  This will be used in , And this is it  

DATA_TRX_ID

, Each record records this transaction  ID, It indicates which transaction modified the current value of this record , Let's get back to business , We know  

Read Uncommitted

,  In fact, isolation is not necessary , Just read the current value directly , here we are  

Read Committed

  Level , We want the transaction to read the committed value ,mysql  Used a call  

read view

  The gadget , It has these values that we should pay attention to ,

m_low_limit_id

,  This is  read view  The largest active transaction at creation  id

m_up_limit_id

,  This is  read view  The smallest active transaction at creation  id

m_ids

,  This is  read view  All active transactions at creation  id  Array

m_creator_trx_id  This is the creation transaction of the current record  id

The main logic for determining the visibility of transactions is as follows ,

• When recorded transactions  
  id
    Less than the minimum active transaction  id, The description is visible ,

• If the recorded transaction  
  id
    Equal to the current transaction  id, The description is your own change , so

• If the recorded transaction  
  id
    Greater than the largest active transaction  
  id
  ,  invisible

• If the recorded transaction  
  id
    Be situated between  
  m_low_limit_id
    and  
  m_up_limit_id
    Between , It is necessary to judge whether it is  
  m_ids
    in , If in , invisible , If not , Indicates that... Has been submitted , See the specific
  Code
  Take a look

/** Check whether the changes by id are visible.
 @param[in] id transaction id to check against the view
 @param[in] name table name
 @return whether the view sees the modifications of id. */
 bool changes_visible(trx_id_t id, const table_name_t &name) const
 MY_ATTRIBUTE((warn_unused_result)) {
 ut_ad(id > 0);

 if (id < m_up_limit_id || id == m_creator_trx_id) {
 return (true);
 }

 check_trx_id_sanity(id, name);

 if (id >= m_low_limit_id) {
 return (false);

 } else if (m_ids.empty()) {
 return (true);
 }

 const ids_t::value_type *p = m_ids.data();

 return (!std::binary_search(p, p + m_ids.size(), id));
 }


What is the rest , Namely  

Read Committed

  and  

Repeated Read

  Are not the same as , That's what I said before  

read view

  Can you support it , How to support it , If the  

read view

  Is created at the beginning of the transaction , That seems to support only  RR  Transaction isolation level , Actually , This is done by creating  

read view

The timing of , about  RR  Level , In the first place of the transaction  

select

  Statement is to create , about  RC  Level , Is at the end of each  

select

  Statements are created once before execution , That will ensure that all submitted data can be read

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The author of this article : 

Nicksxs

 

Creation time : 2020-04-26 

Link to this article : 

I'll explain it to you  MVCC