When does MySQL use table locks and row locks?

Hello everyone , I'm brother Shu .

MySQL Innodb The lock of can be said to be the concurrency basis of the execution engine , Only with locks can the consistency of data be guaranteed . as everyone knows , We all know Innodb There's a global lock 、 Table lock 、 There are three types of row level locks , But you know when to use the watch lock , When will I use the lock ？ Although the MySQL The knowledge points of are quite familiar , But at first I saw this problem , Brother Shu is also a little confused , I really haven't thought about it from this perspective . You can do it for a while 1 Think about the answer in minutes , Later, I will show you how to clarify this problem .

For this question , I can only think of some fragments roughly , for example ：

1. For table level locks , When executed DDL Statement to modify the table structure , Can use watch level lock .
2. For row level locks , Generally, row level locks are used by default , It seems that you need an index to match .

The above is my rough answer , I wonder if the answer you think is the same as me ？ Now let me take you to review MySQL Your lock ！

For databases , The lock range can be divided into ：

• Global lock
• Table lock
• Row-level locks

Global lock

Global lock is to lock the whole database instance . MySQL It provides a way to add global read lock , The order is Flush tables with read lock (FTWRL). When you need to make the entire library read-only , You can use this command , After that, the following statements of other threads will be blocked ： Data update statement （ Data addition, deletion and modification ）、 Data definition statement （ Including building tables 、 Modify table structure, etc ） Commit statements for and update class transactions . You can understand it as , The global lock basically locks all the change statements of the data .

The typical application scenario of global lock is full database logical backup , That is, every table in the whole library select Come out and save . As mentioned above, global lock will lock all change statements , But it's just for MyISAM In terms of storage engines . about Innodb for , It can take advantage of MVCC Realize the consistent view of data , Thus, the data backup of the whole library can be realized without locking the whole library .

Table lock

Table level locks can be divided into ： Table locks 、 Metadata lock 、 There are three kinds of intentional locks .

Table locks

Table locks , As the name suggests, it is to lock a table .

When will the watch lock be used ？

Generally, the corresponding storage engine does not have row level locks （ for example ：MyIASM）, Or corresponding SQL Statement does not match index .

In the first case , Because the corresponding storage engine does not support row locks , Therefore, it can only be achieved by using coarser grained locks , It's easy to understand .

In the second case , If the storage engine supports row locks , But the corresponding SQL No index is used , At this time, the whole table will also be scanned , Then the table lock will also be used at this time . For example, the following statement does not specify a query column , Or the query column is specified but the index is not used , Then the whole table will be locked directly .

Log in and copy

      
      

       
       //  Query column is not specified 
       
       
select * from user;
       
       
//  Specify the query column , But no index is used 
       
       
select * from user where name = 'zhangsan';
      
      
      
      

       
       
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The index mentioned above , It can be said that it is the key to judge whether row level locks will be used . But I have a question ： If the index is used in the query or update , But there are a lot of data to query or update , Of the whole table 80% Even more , At this time, I can use a watch lock , Or a lock ？ It's an interesting question , Interested friends get a test table to verify , We'll talk about this later when we have a chance .

Metadata lock

Metadata , It refers to our table structure and these metadata . Metadata lock （Metadata Lock） Naturally, execution DDL Table structure change statement , We put a lock on the watch .

When will metadata lock, a table level lock, be used ？

When we add, delete, modify, and query a table , Will add MDL Read the lock ; When we want to make changes to the table structure , Will add MDL Write lock .

Intent locks

Intent locks , In essence, it is the product of space for time , It is a thing to improve the efficiency of row lock .

stay InnoDB in , When we lock a record , To improve concurrency , Usually just lock this line of records , Instead of locking the entire table . And when we need to add X When it's locked , We need to traverse the records of the whole table , If every record is not locked , Before you can add X lock . And this traversal process is very time-consuming , At this time, there is the birth of intention lock .

Intention lock is actually to mark whether the table is locked , If a record is locked , Then you must obtain the intention lock of the table . So when we need to judge whether the records of this table are locked , Just judge the intention lock directly , Reduced traversal time , Improved efficiency , It is a typical practice of trading space for time .

So when will intention locks be used ？

It's simple , When locking row records in the table , Will use intention lock .

Row-level locks

time and again , Finally came to the row lock .

What you need to know is , Row level locks are storage engine level locks , Storage engine support is required to be effective . at present MyISAM The storage engine does not support row level locks , and Innodb The storage engine supports row level locking . And global lock 、 Table lock , It is MySQL The level supports the lock .

When will row level locks be used ？

When the addition, deletion, modification and query match the index ,Innodb Can use row level lock .

If there is no match, the index cannot be found , Then table level locks will be used directly .

summary

At the end of the article , Let's review the questions raised at the beginning ：Innodb When to lock the watch , When to use row lock ？

Table level locks include ： Table locks 、 Metadata lock 、 Intent locks .

For watch locks , When the storage engine does not support row level locking , Use the watch lock .SQL When the statement does not match the index , Use the watch lock .

For metadata locks , When adding, deleting, modifying and checking the table , Will add MDL Read the lock . When you make changes to the table structure , Will add MDL Write lock .

For intention lock , When locking row records in a table , Will use intention lock .

For row level locks , When the addition, deletion, modification and query match the index , Can use row level lock .

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