How to avoid full screen scan in mysql?

Methods for MYSQL to avoid full table scans: 1. Create indexes on the columns involved in where and order by; 2. Avoid null value judgments on fields in the where clause, and avoid using "!=" or operations symbol, avoid using or to connect conditions; 3. Use in and not in with caution; 4. Avoid using calculations on index columns.

MYSQL avoids full table scan

1. To optimize the query, try to avoid full table scan. First of all You should consider building indexes on the columns involved in where and order by

2. Try to avoid null value judgments on fields in the where clause, otherwise the engine will give up using the index and perform a full table scan

For example: select id from t where num is null, you can set the default value 0 on num, ensure that there is no null value in the num column in the table, and then query like this: select id from t where num=0

3. Try to avoid using it in the where clause! = or operator, otherwise the engine will give up using the index and perform a full table scan.

4. Try to avoid using or in the where clause to connect conditions, otherwise the engine will give up using the index and perform a full table scan (union can be used)

5.in and Not in should also be used with caution, otherwise it will cause a full table scan (do not use in if you can use between)

6. The following query will also cause a full table scan.

select id from t where name like '%李%', select id from t where name like '%李'

To improve efficiency, you can use this format select id from t where name like '李%', you can also consider full-text search.

7. Avoid using calculations on index columns. In other words, you should try to avoid expression operations and function operations on fields in the where clause. This will cause the engine to give up using the index and perform a full table scan. .

For example: select id from t where num/2=100 should be changed to: select id from t where num=100*2

8. Many times it is a good idea to use exists instead of in. Select: exists is used to check whether the subquery will return at least one row of data. The subquery does not actually return any data, but returns a value of true or false.

select num from a where num in(select num from b)

Replace with the following statement: select num from a where exists (select 1 from b where num=a.num)

9. Do not use select from t anywhere, replace "" with a specific field list, and do not return any unused fields.

10. Use >= instead of>

Efficient: SELECT * FROM EMP WHERE DEPTNO >=4

Inefficient: SELECT * FROM EMP WHERE DEPTNO > 3

The difference between the two is that the former DBMS will jump directly to the first record with DEPT equal to 4, while the latter will first locate the record with DEPTNO=3 and scan forward to the first record with DEPT greater than 3 record of.

11. Replace the having clause with the Where clause

Recommended tutorial: mysql video tutorial

The above is the detailed content of How to avoid full screen scan in mysql?. For more information, please follow other related articles on the PHP Chinese website!