[MySQL] index classification

Briefly understand the various categories of indexes and how indexes are created and deleted .

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One 、 Classification of indexes

MySQL The index of includes the general index 、 Uniqueness index 、 Full-text index 、 Single index 、 Multi column index, spatial index, etc .
● In terms of functional logic , The index mainly includes 4 Kind of , They are ordinary indexes 、 unique index 、 primary key 、 Full-text index .
● According to the physical realization , The index can be divided into 2 Kind of ： Clustered index and non clustered index .
● It is divided according to the number of action fields , Divided into single column index and joint index .

1. General index

When creating a normal index , Without any restrictions , Just to improve query efficiency . This kind of index can be created in any data type , Whether its value is unique and non empty , It is determined by the integrity constraints of the field itself . After indexing , You can query by index , for example , In the table student Field of name Build a general index on , When querying records, you can query according to the index .

2. Uniqueness index

Use UNIQUE Parameter can be set as unique index , When creating a unique index , Limit the value of this index to be unique , But you can have an empty value , There can be multiple unique indexes in a data table .

3. primary key

Primary key index is a special unique index , On the basis of the unique index, a non NULL constraint is added , That is to say NOT NULL+UNIQUE There is at most one primary key index in a table

Why? , This is determined by the physical implementation of the primary key index , Because data stored in files can only be stored in one order .

4. Single index

Create an index on a single field in a table , A single column index is indexed only according to the field , A single column index can be a normal index , It can also be a unique index , It can also be full-text indexing , It is mainly to ensure that the index only corresponds to one field , A table can have multiple single-column indexes .

5. Multiple columns ( Combine 、 union ) Indexes

Multi column index is to create an index on multiple field combinations of a table . The index points to the corresponding fields at the time of creation , You can query through these fields , But only when the first of these fields is used in the query criteria .

for example ： Fields in the table Id name and gender Build a multi column index on idx_id_name_gender, Only fields used in query criteria Id The index will only be used , When using a composite index, follow the leftmost prefix set .

6. Full-text index

7. Add ： Spatial index

Summary ： Different storage engines support different index types :
InnoDB ： Support B-tree、Full-text Wait for the index , I won't support it Hash Indexes ;
MyISAM ： Support B-tree、Full-text Wait for the index , I won't support it Hash Indexes ;
Memory ： Support B-tree、Hash Wait for the index , I won't support it Full-text Indexes ;
NDB ： Support Hash Indexes , I won't support it B-tree、Full-text Wait for the index ;
Archive ： I won't support it B-tree、Hash、Full-text Wait for the index ;