SQL语句常见优化十大案例

但请注意的是：优化只针对SELECT有效，对UPDATE/DELETE子查询无效，固生产环境应避免使用子查询

2.避免函数索引

例：

SELECT * FROM t WHERE YEAR(d) >= 2016;

由于MySQL不像Oracle那样支持函数索引，即使d字段有索引，也会直接全表扫描。
应改为----->

SELECT * FROM t WHERE d >= ‘2016-01-01‘;

3.用IN来替换OR

低效查询

SELECT * FROM t WHERE LOC_ID = 10 OR LOC_ID = 20 OR LOC_ID = 30;

----->
高效查询

SELECT * FROM t WHERE LOC_IN IN (10,20,30);

4.LIKE双百分号无法使用到索引

SELECT * FROM t WHERE name LIKE ‘%de%‘;

----->

SELECT * FROM t WHERE name LIKE ‘de%‘;

5、读取适当的记录LIMIT M,N

SELECT * FROM t WHERE 1;

----->

SELECT * FROM t WHERE 1 LIMIT 10;

6、避免数据类型不一致

SELECT * FROM t WHERE id = ‘19‘;

----->

SELECT * FROM t WHERE id = 19;

7、分组统计可以禁止排序

SELECT goods_id,count(*) FROM t GROUP BY goods_id;

默认情况下，MySQL对所有GROUP BY col1，col2...的字段进行排序。如果查询包括GROUP BY，想要避免排序结果的消耗，则可以指定ORDER BY NULL禁止排序。
----->

SELECT goods_id,count(*) FROM t GROUP BY goods_id ORDER BY NULL;

9、禁止不必要的ORDER BY排序

SELECT count(1) FROM user u LEFT JOIN user_info i ON u.id = i.user_id WHERE 1 = 1 ORDER BY u.create_time DESC;

----->

SELECT count(1) FROM user u LEFT JOIN user_info i ON u.id = i.user_id;

SQL语句常见优化十大案例

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