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Indexes

What is index ？

Index is a method used by storage engine to improve the access speed of database tables data structure .

Advantages and disadvantages of index ？

advantage ：

• Speed up data discovery

• Add an index to the fields used for sorting or grouping , Sure Speed up grouping and sorting

• Speed up the connection between tables

shortcoming ：

• Indexing requires Take up physical space

• It will reduce the efficiency of adding, deleting and modifying tables , Because the table records are added, deleted and modified every time , Need to carry out Dynamic maintenance index , As a result, the addition, deletion and modification time becomes longer

The function of index ？

Data is stored on disk , When querying data , If there is no index , All data will be loaded into memory , Search in turn , More disk reads . With the index , You don't need to load all the data , because B+ The height of the tree is generally 2-4 layer , At most, you only need to read 2-4 Secondary disk , The query speed is greatly improved .

When you need to build an index ？

1. Fields that are often used for queries

2. Often used to index connected fields , Can speed up the connection

3. Fields that often need to be sorted are indexed , Because the index is in order , It can speed up Sorting Query

When not to build an index ？

1. where Fields not used in conditions are not suitable for indexing

2. The table records less

3. It needs to be added, deleted and modified frequently

4. Participate in column calculation The column of is not suitable for indexing

5. The distinction is not high The field of is not suitable for indexing , Such as gender, etc

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