[database] index

Indexing speeds up queries . But the index should be maintained automatically when updating .

Index , It is equivalent to establishing a catalogue for books


The essential difference between primary keys and unique keys is that primary keys are not accepted NULL value , Within the unique key constraint NULL value .
Generally, cluster indexes are automatically created in the primary key column . Data is stored in clustered index order , There can only be one . There can be multiple unique indexes . Often query columns , Index can be created .

By creating an index , In the process of query , Improve system performance
By creating a unique index , It can maintain the uniqueness of each row of data in the database table
When using grouping and sorting clauses for data retrieval , It can reduce the time of grouping and sorting in queries

Creating and maintaining indexes takes time , And the time increases with the amount of data
Indexes need to take up physical space , If you want to build a clustered index , More space is needed
It takes more time to add, delete and modify the data in the table , Because indexes have to be maintained dynamically

scene ：
Often need to search on the column
On the column as the primary key
Often used on linked columns , These columns are mainly some foreign keys , Can speed up the connection
Often need to search by range on the column
Often need to sort on the column
Often used in where In the column above clause