MySQL深入学习——第六章 查询优化&批量导入操作学习笔记
一、SQL优化—优化实战
1、尽量全值匹配
CREATE TABLE `staffs`(
id int primary key auto_increment,
name varchar(24) not null default "" comment'姓名',
age int not null default 0 comment '年龄',
pos varchar(20) not null default "" comment'职位',
add_time timestamp not null default current_timestamp comment '入职时间'
)charset utf8 comment '员工记录表';
insert into staffs(name,age,pos,add_time) values('z3',22,'manage',now());
insert into staffs(name,age,pos,add_time) values('july',23,'dev',now());
insert into staffs(name,age,pos,add_time) values('2000',23,'dev',now());添加索引：
alter table staffs add index idx_staffs_nameAgePos(name,age,pos);
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age = 25;
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age = 25 AND pos = 'dev';当建立了索引列后，能在wherel条件中使用索引的尽量所用。
2、最佳左前缀法则
如果索引了多列，要遵守最左前缀法则。指的是查询从索引的最左前列开始并且不跳过索引中的列。
EXPLAIN SELECT * FROM staffs WHERE age = 25 AND pos = 'dev';
EXPLAIN SELECT * FROM staffs WHERE pos = 'dev';
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';
3、不在索引列上做任何操作
不在索引列上做任何操作（计算、函数、(自动or手动)类型转换），会导致索引失效而转向全表扫描
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';
EXPLAIN SELECT * FROM staffs WHERE left(NAME,4) = 'July';
4、范围条件放最后
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age =22;
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age =22 and pos='manager';
-- 中间有范围查询会导致后面的索引列全部失效：
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age >22 and pos='manager';
5、覆盖索引尽量用
尽量使用覆盖索引(只访问索引的查询(索引列和查询列一致))，减少select * 使用
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age =22 and pos='manager';
EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME = 'July' and age =22 and pos='manager';
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age >22 and pos='manager';
EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME = 'July' and age >22 and pos='manager';
6、不等于要甚用
mysql 在使用不等于(!= 或者<>)的时候无法使用索引会导致全表扫描
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';
EXPLAIN SELECT * FROM staffs WHERE NAME != 'July';
EXPLAIN SELECT * FROM staffs WHERE NAME <> 'July';
如果定要需要使用不等于,请用覆盖索引：
EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME != 'July';
EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME <> 'July';
7、Null/Not 有影响
注意null/not null对索引的可能影响
7.1、自定定义为NOT NULL
EXPLAIN select * from staffs where name is null;
EXPLAIN select * from staffs where name is not null;
在字段为not null的情况下，使用is null 或 is not null 会导致索引失效
解决方式：覆盖索引
EXPLAIN select name,age,pos from staffs where name is not null;
7.2、自定义为NULL或者不定义
创建staffs2表，不设置name属性得默认值，如下所示：
alter table staffs2 add index idx_staffs_nameAgePos(name,age,pos);
alter table staffs2 add index idx_staffs_name(name);执行下面查询语句：
EXPLAIN select * from staffs2 where name is null; -- 带索引查询
EXPLAIN select * from staffs2 where name is not null; -- 无索引查询结果如下如所示：
Is not null 的情况会导致索引失效
解决方式：覆盖索引
EXPLAIN select name,age,pos from staffs where name is not null;
8、Like查询要当心
like以通配符开头('%abc...')mysql索引失效会变成全表扫描的操作
EXPLAIN select * from staffs where name ='july';
EXPLAIN select * from staffs where name like '%july%';
EXPLAIN select * from staffs where name like '%july';
EXPLAIN select * from staffs where name like 'july%';
解决方式：覆盖索引
EXPLAIN select name,age,pos from staffs where name like '%july%';
9、字符类型加引号
字符串不加单引号索引失效:
EXPLAIN select * from staffs where name = 917;
解决方式：覆盖索引
EXPLAIN select name,age,pos from staffs where name = 917
解决方式：请加引号
EXPLAIN select * from staffs where name = '917';
10、OR改UNION效率更高
EXPLAIN select * from staffs where name='July' or name = 'z3';
-- 解决方式：覆盖索引
EXPLAIN select name,age from staffs where name='July' or name = 'z3';
EXPLAIN select * from staffs where name='July' UNION select * from staffs where name = 'z3';
记忆总结：
全职匹配我最爱，最左前缀要遵守；
带头大哥不能死，中间兄弟不能断；
索引列上少计算，范围之后全失效；
LIKE百分写最右，覆盖索引不写*；
不等空值还有OR，索引影响要注意；
VAR引号不可丢， SQL优化有诀窍。
二、批量导入
1、insert语句优化；
- 提交前关闭自动提交
- 尽量使用批量insert语句
- 可以使用MyISAM存储引擎
2、LOAD DATA INFLIE
LOAD DATA INFLIE；
使用LOAD DATA INFLIE比一般的insert语句快20倍
select * into OUTFILE 'D:\\product.txt' from product_info
load data INFILE 'D:\\product.txt' into table product_info
load data INFILE '/soft/product3.txt' into table product_info
show VARIABLES like 'secure_file_priv'
- secure_file_priv 为 NULL 时，表示限制mysqld不允许导入或导出。
- secure_file_priv 为 /tmp 时，表示限制mysqld只能在/tmp目录中执行导入导出，其他目录不能执行。
- secure_file_priv 没有值时，表示不限制mysqld在任意目录的导入导出。
secure_file_priv=''