[Mysql study notes] Advanced optimization --- Mysql lock mechanism

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I. Overview

(1) Definition

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(2) Example introduction: life shopping

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(3) Classification of locks

① From the type of data operation (read, write)

读锁（共享锁）：针对同一份数据，多个读操作可以同时进行而不会互相影响

写锁（排它锁）：当前写操作没有完成前，它会阻断其他写锁和读锁。

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② From the granularity of data operation

• Table lock
• Row lock

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Two and three locks (table, row, page lock)

(1) Table lock (partial reading)★

① Features

偏向MyISAM存储引擎，开销小，加锁快，无死锁，锁定粒度大，发生锁冲突的概率最高，并发最低

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② Case analysis

1. Add/release table lock:

Increase table lock statement

lock table 表名1 read(write), 表名2 read(write) .....;

Release table lock statement

unlock tables;

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2. Add read lock:

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3. Add write lock:

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③ Case conclusion

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④ Table lock analysis

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(2) Row lock (partial writing)★

① Features

偏向InnoDB存储引擎，开销大，加锁慢；会出现死锁；锁定粒度最小，发生锁冲突的概率最低，并发度也最高。

InnoDB与MyISAM的最大不同有两点：
一是支持事务（TRANSACTION）;
二是采用了行级锁

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② Row lock support transaction (review old knowledge)

1. Transaction (Transation) and its ACID attributes

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2. Problems caused by concurrent transaction processing

• Update loss (Lost Update)
  

• Dirty read (Dirty Reads)
  

• Non-repeatable read (Non-Repeatable Reads)
  

• Magic Reading (Phantom Reads)
  

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3. Transaction isolation level

查看事务的隔离级别：show variables like "%tx_isolation%";

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③ Case analysis

1. Basic demonstration of row lock

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2. No index row lock is upgraded to table lock

varchar  不用 ' '  导致系统自动转换类型, 行锁变表锁

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3. Gap lock hazards

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4. Interview questions: how to lock a line for the regular exam

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④ Case conclusion

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⑤ Row lock analysis

检查innodb的行锁状态：show status like "%innodb_row_lock%";

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⑥ Optimization suggestions

• As far as possible, all data retrieval is completed through the index , to avoid the upgrade of non-indexed row locks to table locks
• Design the index reasonably to minimize the scope of the lock
• Possible to reduce the search condition , to avoid gaps lock
• Try to control the size of the transaction , reduce the amount of locked resources and the length of time
• Low-level transaction isolation possible

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(3) Page lock (understand)

开销和加锁时间界于表锁和行锁之间：会出现死锁；

锁定粒度界于表锁和行锁之间，并发度一般。

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(4) How to choose three locks?