Introduction to four MySQL engines

（1）：MyISAM Storage engine
Unsupported transaction 、 Foreign keys are also not supported , The advantage is fast access , There is no... For transaction integrity Ask for or with select,insert The main application can basically use this engine to create tables

Support 3 Different storage formats , Namely ： Static table ; Dynamic table ; Compression meter

Static table ： The fields in the table are all non variable length fields , So every record is a fixed length , Advantages 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 （ Because the space will be filled according to the width of the column definition when storing ）ps： When I get the data , By default, the space after the field is removed , If you don't pay attention, you will ignore the spaces in the data itself .

Dynamic table ： Records are not fixed length , The advantage of this storage is that it takes up less space ; shortcoming ： Frequent updates 、 Deleting data is prone to fragmentation , It needs to be carried out on a regular basis OPTIMIZE TABLE perhaps myisamchk-r Command to improve performance

Compression meter ： Because each record is compressed individually , So there's only a very small cost of access

（2）InnoDB Storage engine *

The storage engine provides 、 Transaction security of rollback and crash recovery capability . But contrast MyISAM engine , The efficiency of writing will be lower , And it will take up more disk space to keep data and index . 
InnoDB Features of the storage engine ： Support auto grow Columns , Support for foreign key constraints

(3)：MEMORY Storage engine

Memory The storage engine uses what is in memory to create tables . Every memory Table only corresponds to one disk file , The format is .frm.memory Table access of type is very fast , Because its data is in memory , And by default HASH Indexes , But once the service is shut down , The data in the table will be lost . 
MEMORY The table of the storage engine can choose to use BTREE Index or HASH Indexes , Two different types of indexes have different scopes of use

Hash Indexing benefits ： 
Hash The particularity of index structure , Its retrieval efficiency is very high , The index can be retrieved at one time , Unlike B-Tree The index needs to be from the root node to the branch node , Finally, you can access the page node for many times IO visit , therefore Hash The query efficiency of index is much higher than B-Tree Indexes . 
Hash Indexing disadvantages ： What about imprecise search , Is also evident , because hash The algorithm is based on equivalent calculation , So for “like” Equal range search hash Invalid index , I won't support it ;

Memory Type of storage engine is mainly used for code tables whose content changes infrequently , Or as an intermediate result table for statistical operations , It is convenient to analyze the intermediate results efficiently and get the final statistical results ,. For the storage engine memory Be careful when updating tables , Because the data is not actually written to disk , So we must consider how to get the modified data after the next service restart .

（4）MERGE Storage engine

 

Merge The storage engine is a set of MyISAM Combination of tables , these MyISAM Tables must be exactly the same structure ,merge The table itself has no data , Yes merge Types of tables can be queried , to update , Delete operation , These operations are actually internal MyISAM Table .