MySQL practice 45 [global lock and table lock]

06 | Global lock and table lock ： How can adding a field to a table be so much of a hindrance ？

The original intention of database lock design is to deal with concurrent problems . Shared resources as multiple users , When concurrent access occurs , Database needs to control access rules of resources reasonably . Lock is an important data structure to implement these access rules .

According to the range of locking ,MySQL The locks inside can be roughly divided into global locks 、 Table level lock and row lock

Global lock
Global lock is to lock the whole database instance
FTWRL

Flush tables with read lock

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 .

Table lock
MySQL There are two types of lock at the inner table level ： One is watch lock , One is metadata lock （meta data lock,MDL)

 The syntax of table lock is 
lock tables  Table name  read/write

 It can be used  unlock tables  Active release lock , It can also be released automatically when the client is disconnected .
 We need to pay attention to ,lock tables  The syntax will restrict the reading and writing of other threads ,
 It also defines the next operation objects of this thread 

for instance , If in a thread A In the implementation of lock tables t1 read, t2 write; This statement , Then other threads
Write t1、 Reading and writing t2 All of the statements will be blocked . meanwhile , Threads A In execution unlock tables Before , Can only hold
Line reading t1、 Reading and writing t2 The operation of . Linking t1 Not allowed , Naturally, you can't access other tables

Table level lock is MDL（metadata lock)
MDL You don't need to explicitly use , When accessing a table, it will be automatically added .MDL The role of is , Ensure the correctness of reading and writing .
stay MySQL 5.5 The version introduces MDL,
When adding, deleting, modifying and querying a table , Add MDL Read the lock ;
When you want to make structural changes to a table , Add MDL Write lock

Read locks are not mutually exclusive , So you can have multiple threads to add, delete, modify and query a table at the same time , Read-write lock 、 Write locks are mutually exclusive , To ensure the security of the operation to change the structure of the table . therefore , If two threads want to add fields to a table at the same time , One of them can't be executed until the other has finished

Add fields to a table , Or modify the fields , Or the Caucasian , Need to scan the data of the whole table

In the transaction MDL lock , Apply at the beginning of statement execution , However, the statement is not released immediately after completion , It will wait until the whole transaction is committed before releasing

stay MySQL Of information_schema Library innodb_trx In the table , You can find the current transaction in progress . If you want to
do DDL The changed table happens to have a long transaction running , Consider suspending DDL, perhaps kill Drop this long business .

stay alter table Set the waiting time in the statement , If you can get it within the specified waiting time MDL It's better to write lock , Take no
Do not block the following business statements , Give up first . Then the developer or DBA Then repeat this through the retry command
A process .

ALTER TABLE tbl_name NOWAIT add column ...
ALTER TABLE tbl_name WAIT N add column ...

Online DDL The process is like this ：

1. take MDL Write lock
2. Drop into MDL Read the lock
3. Really doing DDL
4. Upgrade to MDL Write lock
5. Release MDL lock
   1、2、4、5 If there are no lock conflicts , Execution time is very short . The first 3 Step to take up the DDL Most of the time , During this period, this watch
   Data can be read and written normally , So it's called “online ”

Reference resources

MySQL in lock tables and unlock tables elementary analysis