MySQL storage engine

Preface

If you want to be right SQL To optimize , Then understanding the storage engine is essential . Thinking about that, write an article about the engine , It's also a review of yourself

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1、 View storage engine

Before understanding the storage engine , We must know MySQL Which engines are provided to us , adopt ：show engines see

2、 Set the default storage engine of the system

We can check the default storage engine first

show variables like '%storage_engine%';
# or 
SELECT @@default_storage_engine;

my MySQL Version of 8.x, You can see that the default engine is InnoDB

How to modify the default storage engine ？

If it is 8.0 And above , If the storage engine of the table is not explicitly specified in the statement of creating the table , That will default to InnoDB As a storage engine for tables .
If it is 5.x, If we want to change the default storage engine of the table , You can write the command line to start the server like this ：

SET DEFAULT_STORAGE_ENGINE=MyISAM;

Or modify my.cnf file ：

default-storage-engine=MyISAM
#  Restart the service 
systemctl restart mysqld.service

3、 Set the storage engine for the table

The storage engine is responsible for extracting and writing the data in the table , We can Different tables set different storage engines , In other words, different tables can have different physical storage structures , Different extraction and writing methods .

3.1、 Specify the storage engine when creating the table

If the statement that creates the table does not specify the storage engine of the table , Then the default storage engine will be used InnoDB . If we want to explicitly specify the storage engine of the table , It can be written like this ：

CREATE TABLE  Table name (
 Create table statement ;
) ENGINE =  Storage engine name ;

3.2、 Modify the storage engine of the table

If the table has been built , We can also use the following statement to modify the storage engine of the table ：

ALTER TABLE  Table name  ENGINE =  Storage engine name ;

For example, let's modify engine_demo_table Table storage engine ：

ALTER TABLE engine_demo_table ENGINE = InnoDB;

4、 Engine introduction

good , Here we are. , Because it is completely familiar to everyone SQL Statement first understand the engine , I believe you have a little feeling about the engine , If you haven't felt , No problem , Next, I will only focus on ：

4.1、InnoDB engine ： Transaction storage engine with foreign key support

• MySQL from 3.23.34a Start with InnoDB Storage engine . Greater than or equal to 5.5 after , By default InnoDB engine .

• InnoDB yes MySQL Of Default transactional engine , It's designed to handle a lot of short-term (short-lived) Business . It can ensure the complete submission of transactions (Commit) And rollback (Rollback).

• In addition to adding and querying , It needs to be updated 、 Delete operation , that , Preference should be given to InnoDB Storage engine .

• Unless there's a very special reason to use another storage engine , Otherwise, priority should be given to InnoDB engine .

• Data file structure ：
  （1） Table name .frm Storage table structure
  （2） Table name .ibd Store data and index

• contrast MyISAM Storage engine for , InnoDB The processing efficiency of writing is poor , And it will take up more disk space to save data and indexes .

• MyISAM Cache index only , Don't cache real data ;InnoDB Not only index but also real data , High memory requirements , And memory size has a decisive impact on performance .

4.2、MyISAM engine ： The main non transactional storage engine

• MyISAM There are a lot of features , Include full text index 、 Compress 、 Space function (GIS) etc. , but MyISAM Unsupported transaction 、 Row-level locks 、 Foreign keys , There is no doubt that the defect is Can't recover safely after a crash .（ This is also caused by not supporting transactions ）
• MySQL5.5 The previous default storage engine
• Advantage is Access to the Fast , There is no requirement for transaction integrity or SELECT、INSERT Mainly applications
• There is additional constant storage for data statistics . so count(*) The query efficiency is very high
• Data file structure ：
  （1） Table name .frm Storage table structure
  （2） Table name .MYD Store the data
  （3） Table name .MYI Storage index
• Application scenarios ： Read only applications or read-only businesses

4.3、Archive engine ： For data archiving

4.4、Memory engine ： Table in memory

summary ：
Memory The logical medium used is Memory , Fast response , however When mysqld When the daemon crashes Data will be lost . in addition , The stored data is required to be in a format with constant data length , such as ,Blob and Text Data of type is not available ( Of variable length ).

Main features ：

• Memory meanwhile Support hash （HASH） Indexes and B+ Tree index .
• Memory The watch is at least better than MyISAM Table One order of magnitude fast .（MylSAM Than InnoDB Be quick , however Memory Better than MylSAM fast , Because it is read directly from memory ）
• MEMORY The size of the table is limited Of . The size of the table mainly depends on two parameters , Namely max_rows and max_heap_table_size . among ,max_rows You can specify... When creating a table ;max_heap_table_size The default size of is 16MB, It can be expanded as needed .
• Data files are stored separately from index files
• shortcoming ： Its data is easy to lose , Short life cycle . Based on this flaw , choice MEMORY You need to be very careful when storing the engine .

Memory Usage scenarios of storage engine ：

• The target data is relatively small , And very Frequent visits , Store data in memory , If the data is too large, it will cause out of memory . You can use the parameter max_heap_table_size control Memory The size of the table , Limit Memory The maximum size of the table .
• If The data is provisional , and Must be available immediately obtain , Then you can put it in memory .
• Stored in Memory In the table If the data suddenly If you lose it, it doesn't matter much .

4.5、 Other engines

MySQL The storage engine and ：

• Blackhole engine ： Discard write operation , The read operation will return empty content
• CSV engine ： When storing data , Separate data items with commas
• Federated engine ： Access remote tables
• Merge engine ： Manage multiple MyISAM A collection of tables made up of tables
• NDB engine ：MySQL Cluster specific storage engine

These are not commonly used , But also list , Interested partners can learn about it alone .

MyISAM and InnoDB Comparison of

MySQL5.5 The previous default storage engine was MyISAM,5.5 Then it changed to InnoDB.

MySQL In the same database , Different storage engines can be selected for different tables . And in the real world , The two most used engines are MylSAM and InnoDB 了 . Here's a point to note , Is not to say that InnoDB As MySQL5.5 After that, the default engine must be better than MylSAM The engine is good , There is no saying that InnoDB To replace MyISAM Of , The two are more inclined to a complementary relationship .

Contrast item	MylSAM	InnoDB
Foreign keys	I won't support it	Support
Business	I won't support it	Support
Row table lock	Table locks , Even operating on one record locks the entire table , Not suitable for highly concurrent operations	Row lock , Lock only one line during operation , It doesn't affect anything else , Suitable for high concurrency operations
cache	Cache index only , Don't cache real data	Not only index but also real data , High memory requirements , And memory size has a decisive impact on performance
Use the self-contained system table	Y	N
concerns	performance ： Save resources 、 Less consumption 、 Suitable for simple business	Business 、 Write concurrently 、 More resources
Default installation	Y	Y
By default	N	Y

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summary

If there is anything wrong , I hope you can point out that , thank .