MySQL storage engine and its differences

MySQL 5.7 The supported storage engines are InnoDB、MyISAM、Memory、Merge、Archive、Federated、CSV、BLACKHOLE etc. .

have access to SHOW ENGINES Statement to view the engine types supported by the system , The result is shown in the figure .

difference

1.myisam Is the default table type and is not transaction safe ;innodb Support transactions .

2.myisam Foreign key not supported ;Innodb Support foreign keys .

3.myisam Support table level lock （ High concurrency is not supported , Mainly reading ）;innodb Support row lock （ Shared lock , Exclusive lock , Intent locks ）, Smaller particle size , But when performing a scan that cannot determine the scope sql When the sentence is ,innodb It will also lock the whole watch .

4. Execute a lot of select,myisam Is the best choice ; Perform a lot of update and insert It is best to innodb.

5.myisam There are three files stored on disk .frm( Storage table definition ) .myd（ Store table data ） .myi（ Storage table index ）;innodb What is stored on disk are tablespace data files and log files ,innodb The size of the table is limited only by the size of the operating system .

6.myisam Use nonclustered indexes , Index and data are separated , Cache index only ;innodb Using a clustered index , Index and data exist in a file .

7.myisam Save the specific number of rows in the table ;innodb Do not save .

8.delete from table when ,innodb Will not re resume , It will be deleted line by line .

myisam

Index by B+ Trees make up , When executing the query operation, you will first search B+ Trees , If the corresponding leaf node is found, it will , According to the value of leaf node （ Address ）, Take out the whole row of data .

innodb

Above, InnoDB Main index （ It's also a data file ） Schematic diagram , You can see that the leaf node contains the complete data record . This kind of index is called clustered index . because InnoDB The data files themselves should be aggregated by primary key , therefore InnoDB Table must have primary key （MyISAM There can be no ）, If not explicitly specified , be MySQL The system will automatically select a column that can uniquely identify the data record as the primary key , If there is no such column , be MySQL Automatically for InnoDB Table generates an implicit field as the primary key , The length of this field is 6 Bytes , The type is long plastic .

It's in English ASCII Code as a comparison criterion . The implementation of clustered index makes the search by primary key very efficient , But the secondary index search needs to retrieve the index twice ： First retrieve the secondary index to get the primary key , Then use the primary key to retrieve the record in the primary index .