MySQL5.5 In the future, it will be used by default InnoDB Storage engine , among InnoDB and BDB Provide transaction safety watch , Other storage engines are non transactional security tables .
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 .
The main storage engine :MyISAM、InnoDB、MEMORY and MERGE Introduce :
When creating a table, use engine=... or type=... To specify the engine to use .show table status from DBname To view the specified table to the engine .

( One )MyISAM
  
It does not support transactions , Foreign keys are also not supported , Especially fast access , There is no requirement for transaction integrity or SELECT、INSERT The main application can basically use this engine to create tables .
Every MyISAM Stored on disk as 3 File , Where file name and table name are the same , But the extensions are :

  • .frm( Storage table definition )
  • MYD(MYData, Store the data )
  • MYI(MYIndex, Storage index )

   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 .
  MyISAM The table also supports 3 Different storage formats :

  • static state ( Fixed length ) surface
  • Dynamic table
  • Compression meter

   The static table is 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 . 
( Two )InnoDB
  
InnoDB The storage engine provides with commit 、 Transaction security of rollback and crash recovery capability . But contrast MyISAM Storage engine for ,InnoDB Write processing is less efficient and takes up more disk space to retain data and indexes .
1) Auto grow Columns :
  
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 .
2) Foreign key constraints :
  
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 ,set foreign_key_checks=1; Open constraints .
( 3、 ... and )MEMORY
  
memory Use what's in memory to create a 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 .
      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 the same 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 .

create table tab_memory engine=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  hash on  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 .
   The server needs enough memory to keep the server in use at the same time MEMORY surface , When not in use MEMORY Table time , To release MEMORY Memory occupied by the table , Should carry out DELETE FROM or truncate table Or delete the entire table .
   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 .
( Four )MERGE
  
merge The storage engine is a set of MyISAM Combination of tables , these MyISAM The table structure must be exactly the same ,MERGE There is no data in the table , Yes MERGE Types of tables can be queried 、 to update 、 Deleted actions , These operations are actually internal MyISAM Table operation . For MERGE Table insert operations , It's based on INSERT_METHOD Clause defines the inserted table , There can be 3 Different values ,first and last Value causes the insert operation to be applied to the first or last table , Do not define this clause or NO, That means you can't do this MERGE Table to insert . It can be done to MERGE table drop operation , This operation is just deleting MERGE The definition of the table , No impact on internal tables .MERGE Keep... On disk 2 In a MERGE Table name start file :.frm Definition of file storage table ;.MRG The file contains information about the composite table , Include MERGE Which tables are made up of , The basis for inserting data . It can be modified by .MRG File to modify MERGE surface , But after modification, we have to pass flush table Refresh .

create  table  man_all(id int , name  varchar (20))engine=merge union =(man1,man2) insert_methos= last ;

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