What is the storage engine and the three common database storage engines for MySQL

Reprint ： What is a storage engine (Save Engines)？

The storage engine is how to store data 、 How to index and update stored data 、 Query data and other technologies . Because in a relational database, data is stored in the form of tables , So a storage engine can also be called a table type （ That is, the type of storage and operation of this table ）.

stay Oracle and SQL Server There is only one storage engine in the database , All data storage management mechanisms are the same . and MySql The database provides a variety of storage engines . Users can choose different storage engines for data tables according to different requirements , Users can also write their own storage engine according to their own needs .

 

1、MySql What are the storage engines in ？

1. MyISAM： This kind of engine is mysql The first . This kind of engine can be divided into static state MyISAM、 dynamic MyISAM And compression MyISAM Three , Whatever it is MyISAM surface , At present, it doesn't support transactions , The function of row level lock and foreign key constraint .
2. MyISAM Merge engine ： This type is MyISAM A variant of the type . Merging tables is to combine several of the same MyISAM Tables merged into a virtual table . It is often used in log and data warehouse .
3. InnoDB：InnoDB A table type can be seen as a pair of MyISAM Further update products , It provides transactions 、 The function of row level lock mechanism and foreign key constraint , Also present MySQL The default storage engine .
4. Memory(heap)： This type of data table only exists in memory . It uses hash index , So data access is very fast . Because it's in memory , So this type is often used in temporary tables .
5. archive： This type only supports select and insert sentence , And it doesn't support indexing . It is often used in logging and aggregation analysis .

Of course MySql More than the above table types are supported . Let's introduce how to view and set the data table type .

 

2、 Storage engine operations

Look at the storage engines that the database can support , use show engines; The command can display the storage engines supported by the current database ：

--  Inquire about  MySQL  Supported storage engines 
SHOW ENGINES\G;

--  Query results ( Omitted results )

***************************[ 2. row ]***************************
Engine       | MEMORY
Support      | YES
Comment      | Hash based, stored in memory, useful for temporary tables
Transactions | NO
XA           | NO
Savepoints   | NO
***************************[ 5. row ]***************************
Engine       | MyISAM
Support      | YES
Comment      | MyISAM storage engine
Transactions | NO
XA           | NO
Savepoints   | NO

........

MySQL from 5.0 At first, the default storage engine is InnoDB, This can be done through the system variable ​default_storage_engine​  Query the default storage engine of the system .

mysql [email protected]:sakila> show variables like 'default_storage_engine';
+------------------------+--------+
| Variable_name          | Value  |
+------------------------+--------+
| default_storage_engine | InnoDB |
+------------------------+--------+

The storage engine is table based , So you can specify the storage engine at the time of creation .

--  Specify the storage engine when creating the table , The default is  InnoDB
CREATE TABLE test_table(
id int primary key  auto_increment,
name varchar(128) NOT NULL
)ENGINE  = MyISAM;

Reprint ：MySQL - Three common database storage engines

Let's take a look at some of the common engines .

1.InnoDB Storage engine

InnoDB Is the preferred engine for transactional databases , Support transaction security （ACID）, Other storage engines are non transactional security tables , Support row locking and foreign keys ,MySQL5.5 In the future, it will be used by default InnoDB Storage engine .

InnoDB The main features

by MySQL Provided with commit 、 Transaction security of rollback and crash recovery capability （ACID compatible ） Storage engine .InnoDB Locked at line level and also at SELECT Statement provides a similar Oracle Non locked reading . These capabilities add to multi-user deployment and performance . stay SQL Querying , Be free to InnoDB Types of watches and other MySQL The table types are mixed , Even in the same query can be mixed .

InnoDB The auto growing columns of the table can be inserted manually , But if it's empty or 0, The actual insertion is automatically increased to the value . Can pass "ALTER TABLE...AUTO_INCREMENT=n;" Statement to force the start value of the autogrow value , The default is 1, But the force to the default value is stored in memory , This value will be lost when the database is restarted . have access to LAST_INSERT_ID() Query the value used by the current thread to insert the record last . If you insert more than one record at a time , Then the auto growth value used by the first record is returned .

about InnoDB surface , Autogrow columns must be indexes . If it's a composite index , It must also be the first column of the composite index , But for MyISAM surface , Autogrow columns can be other columns of a composite index , After inserting the record like this , The auto growing columns are sorted and incremented according to the combined index .

MySQL The only storage engine that supports foreign keys is InnoDB, When creating a foreign key , The parent table must have a corresponding index , When creating a foreign key, the sub table will also automatically create the corresponding index . When creating an index , Can be specified in delete 、 When updating the parent table , The corresponding operation on the sub table , Include restrict、cascade、set null and no action. among restrict and no action identical , It refers to the restriction when the sub tables are related , The parent table cannot be updated ;casecade Indicates that when the parent table is updated or deleted , Update or delete the record corresponding to the sub table ;set null When the parent table is updated or deleted , The fields corresponding to the sub table are set null. When a table is referenced by a foreign key created by another table , Then the index or primary key corresponding to the table is forbidden to be deleted .

have access to set foreign_key_checks=0; Temporarily turn off foreign key constraints ,setforeign_key_checks=1; Open constraints .InnoDB The storage engine maintains its own buffer pool for caching data and indexes in main memory .InnoDB Put its tables and indexes in a logical table space , A table space can contain several files （ Or the original disk file ）. This is related to MyISAM The watch is different , For example MyISAM Each table in the table is stored in a separate file .InnoDB A watch can be any size , Even when the file size is limited to 2GB On the operating system of .

InnoDB Support for foreign key integrity constraints , When storing data in a table , Each table is stored in primary key order , If the primary key is not specified when the table is defined ,InnoDB A... Will be generated for each line 6 Bytes of ROWID, And use this as the primary key .

Use InnoDB Storage engine MySQL A file named... Will be created in the data directory ibdata1 Of 10MB Automatic extended data file size , And two named ib_logfile0 and ib_logfile1 Of 5MB The size of the log file

2.MyISAM Storage engine

MyISAM be based on ISAM Storage engine , And expand it . It's in Web、 One of the most commonly used storage engines in data warehousing and other application environments .MyISAM Has a higher insert 、 Query speed , Transaction is not supported , Foreign key not supported .

MyISAM The main features ：

Supported by large file systems and operating systems .

When you mix delete with update and insert , Dynamic sized rows produce less fragmentation . This is done by merging adjacent deleted blocks , If the next block is deleted , Expand to the next piece of autocomplete .

Every MyISAM The maximum number of indexes in the table is 64, This can be changed by recompiling . The maximum number of columns per index is 16.

The maximum bond length is 1000 byte , This can also be changed by compiling , For keys longer than 250 Byte case , One is more than 1024 Byte keys will be used .

BLOB and TEXT Columns can be indexed .

NULL Allowed in indexed columns , This value takes up... Of each key 0~1 Bytes .

All numeric key values are stored in high byte priority to allow a higher index compression .

Every MyISAM All types of tables have a AUTOINCREMENT Internal columns of , When INSERT and UPDATE This column is updated during the operation , meanwhile AUTOINCREMENT The columns will be refreshed . So ,MyISAM Of the type table AUTOINCREMENT Column update ratio InnoDB Type of AUTOINCREMENT faster .

Data files and index files can be placed in different directories , Average distribution IO, Get faster . To specify the path of the data file and index file , You need to create a table through DATA DIRECTORY and INDEX DIRECTORY The statement specifies , File paths need to use absolute paths .

Every MyISAM Every watch has a sign , Server or myisamchk The program is checking MyISAM This flag is set when the data table is created .MyISAM The table also has a flag to indicate whether the data table has been closed normally since it was last used . If the server thinks it's down or crashed , This flag can be used to determine whether the data table needs to be checked and repaired . If you want to make this check automatic , You can use it when you start the server --myisam-recover The phenomenon . This allows the server to open one at a time MyISAM Data table is the mark of automatically checking data table and repairing it if necessary .MyISAM Tables of type can be corrupted , have access to CHECK TABLE Statement to check MyISAM Table health , And use REPAIR TABLE Statement to fix a corruption to MyISAM surface .

Each character column can have a different character set .

Yes VARCHAR The table can be fixed or dynamic record length .

VARCHAR and CHAR Columns can be up to 64KB.

Use MyISAM The engine creates the database , Will produce 3 File . The name of the file starts with the name of the table , File type at extension ：frm File storage table definition 、 The extension of the data file is .MYD（MYData）、 When indexing file extensions .MYI（MYIndex）.MyISAM Table support 3 Different storage formats ： static state ( Fixed length ) surface , Dynamic table , Compression meter .

Static tables are the default storage format . Fields in static tables are all non variable length fields , So every record is a fixed length , The advantage of this storage method is that the storage is very fast , Easy to cache , It's easy to recover in case of failure ; The disadvantage is that it usually takes up more space than a dynamic table . The static table will complement the space according to the width defined by the column when storing data , But you don't get these spaces when you visit , These spaces have been removed before being returned to the application . And also notice that ： In some cases, you may need to return the space after the field , When this format is used, the space after it will be automatically processed .

Dynamic tables contain variable length fields , Records are not fixed length , The advantage of this storage is that it takes up less space , However, frequent updating and deletion of records will cause fragmentation , It needs to be carried out on a regular basis OPTIMIZE TABLE Sentence or myisamchk -r Command to improve performance , And it's relatively difficult to recover in case of failure .

The compressed table consists of myisamchk Tool creation , Take up a very small space , Because each record is compressed individually , So there's only a very small cost of access .

3.MEMORY Storage engine

MEMORY The storage engine stores the data in the table in memory , Provides quick access to queries and references to other table data .

MEMORY The main features ：

MEMORY A watch can have as many as 32 An index , Every index 16 Column , as well as 500 The maximum key length of bytes .

Can be in a MEMORY There is not only key value in the table .

MEMORY Support AUTO_INCREMENT Columns and pairs can contain NULL Index of the column of the value .

MEMORY Tables are shared between all clients （ Like anything else TEMPORARY surface ）.

MEMORY Table memory is stored in memory , Memory is MEMORY Tables and servers are idle during query processing , Internal table share created .

By default ,MEMORY Tables use hash indexes , Use this index to “ Equal comparison ” Very fast , But yes. “ Range comparison ” It's much slower . therefore , Hash index values are suitable for "=" and "<=>" In the operator of , Not suitable for use in "<" or ">" In the operator , It's also not suitable for order by In words and sentences . If you really want to use "<" or ">" or betwen The operator , have access to btree Index to speed up .

Stored in MEMORY The data rows in the data table are in a fixed length format , So speed up processing , This means that you can't use BLOB and TEXT Such a variable length data type .VARCHAR It's a variable length type , But because it's in MySQL The interior is fixed in length CHAR type , So you can use it .

create table tab_memoryengine=memory select id,name,age,addr from man order by id;

Use USING HASH/BTREE To specify a specific index .

create index mem_hash using hashon tab_memory(city_id);

Start up MySQL Use when serving --init-file Options , hold insert into...select or load data infile Such statements are put into this file , You can load tables from persistent data sources when the service starts .

Every MEMORY The amount of data placed in the table , suffer max_heap_table_size Constraints on system variables , The initial value of this system variable is 16M, At the same time, creating MEMORY You can use MAX_ROWS Clause to specify the maximum number of rows in the table .

Every MEMORY The table actually corresponds to a disk file , The format is .frm.MEMORY Type of table access is very fast , Because the data is in memory , And by default HASH Indexes , But once the server is shut down , The data in the table will be lost , But the table will continue to exist .　 The server needs enough memory to keep the server in use at the same time MEMORY surface , When there is no need for MEMORY The contents of the table , To release MEMORY Memory used by table , Should carry out DELETE FROM or TRUNCATE TABLE, Or delete the entire table （ Use DROP TABLE）.

4. Choice of storage engine

In practice , Choosing a suitable storage engine is a complicated problem . Each storage engine has its own advantages and disadvantages , We can't generally say who is better than who .

Storage engine comparison

InnoDB： Support transaction processing , Support foreign keys , Support for crash recovery and concurrency control . If you need to have a high requirement for transaction integrity （ Such as banks ）, Concurrent control is required （ Like selling tickets ）, Then choose InnoDB There are great advantages . If frequent updates are needed 、 Delete the database of operation , You can also choose InnoDB, Because it supports transaction commit （commit） And rollback （rollback）.

MyISAM： Insert data fast , Low space and memory usage . If the table is mainly used to insert new records and read records , So choose MyISAM It can achieve high processing efficiency . If the integrity of the application 、 Concurrency requirements are relatively low , You can also use .

MEMORY： All the data is in memory , Data processing speed is fast , But the security is not high . If you need fast reading and writing speed , Low security requirements for data , You can choose MEMOEY. It has requirements for the size of the table , Can't build too large a table . therefore , This kind of database is only used in relatively small database tables .

The same database can also use a variety of storage engine tables . If a table requires high transaction processing , You can choose InnoDB. In this database, you can select tables with high query requirements MyISAM Storage . If the database needs a temporary table for queries , You can choose MEMORY Storage engine .

To modify the default engine , You can modify... In the configuration file default-storage-engine. Can pass ：show variables like 'default_storage_engine'; View the current database to the default engine . command ：show engines and show variables like 'have%' You can list the engines supported by the current database . among Value Is shown as disabled Indicates that the database supports this engine , It is disabled when the database is started . stay MySQL5.1 in the future ,INFORMATION_SCHEMA There is a... In the database ENGINES Table of , The information it provides is similar to show engines; The sentence is exactly the same , You can use the following statement to query which storage engines support transaction processing ：select engine from information_chema.engines where transactions ='yes';

Can pass engine Keyword specifies the engine used when creating or modifying a database .

When creating a table, use engine=... or type=... To specify the engine to use .show table status from DBname To view the engine of the specified table .