One 、 What are the indicators to measure query overhead

1. response time
2. Number of scanning lines
3. Number of rows returned

Two 、 What factors lead to slow query speed

1. Search for unnecessary records .
2. Returns all columns when the table is associated
3. Always take out all columns

3、 ... and 、 How to optimize

1. Use index
The easiest and fastest way to get results is to give your conditions （ primary key , General index , Unique index, etc ）.
2、 Use joins instead of subqueries
3、 Use federated queries instead of creating temporary tables
4、 Business
5、 Using the foreign key
6、 Optimize the query method
Examples of optimizing query statements