MySQL index

Indexes

Quickly find data structures in order

Point to data as a pointer

Index method ：

Concept

Beyond the data itself , The database also maintains a data structure that satisfies a specific search algorithm , These data structures point to data in some way , In this way, advanced search algorithms can be realized on the basis of these data structures , This data structure is the index .

Advantages and disadvantages

advantage ：

• Improve the efficiency of data retrieval , Reduce the IO cost
• Reduce the sorting cost of data , To reduce the CPU Consumption of

Inferiority ：

• Occupancy space
• Reduce the update speed of the table
• Indexing takes time

classification

Single value index

An index contains only a single column , A table can have multiple single-column indexes

The maximum number of indexes for a table should not exceed 5 individual

unique index

The value of the index column must be unique , But you can have an empty value

Composite index

An index contains multiple columns

Basic grammar

Index structure

• BTree Indexes
• Hash Indexes
• full-text Full-text index
• R-Tree Indexes

Suitable for indexing

• Primary key automatically creates unique index
• Fields that are frequently used as query criteria should be indexed
• Fields associated with other tables in the query , Index foreign key relationship
• Frequently updated fields are not suitable for index creation （ Updating requires not only updating the data but also updating the index ）
• WHERE Fields that cannot be used by the condition are not indexed
• Single key / The choice of Composite Index , High and low tendency to create composite index
• Fields sorted in the query , If the sorting field is accessed by index, the sorting speed will be greatly improved
• Statistics or grouping fields in query

Not suitable for indexing

• There are too few records
• Often add, delete, or alter tables
• Duplicate and evenly distributed table fields