学员表插50万条, 班级表插1万条。
建表
CREATE TABLE `class` (
`id` INT ( 11 ) NOT NULL AUTO_INCREMENT,
`className` VARCHAR ( 30 ) DEFAULT NULL,
`address` VARCHAR ( 40 ) DEFAULT NULL,
`monitor` INT NULL,
PRIMARY KEY ( `id` )
) ENGINE = INNODB AUTO_INCREMENT = 1 DEFAULT CHARSET = utf8;
CREATE TABLE `student` (
`id` INT ( 11 ) NOT NULL AUTO_INCREMENT,
`stuno` INT NOT NULL,
`name` VARCHAR ( 20 ) DEFAULT NULL,
`age` INT ( 3 ) DEFAULT NULL,
`classId` INT ( 11 ) DEFAULT NULL,
PRIMARY KEY ( `id` ) #CONSTRAINT `fk_class_id` FOREIGN KEY (`classId`) REFERENCES `t_class` (`id`)
) ENGINE = INNODB AUTO_INCREMENT = 1 DEFAULT CHARSET = utf8;
设置参数
set global log_bin_trust_function_creators=1; # 不加global只是当前窗口有效。
创建函数
保证每条数据都不同。
#随机产生字符串
DELIMITER //
CREATE FUNCTION rand_string ( n INT ) RETURNS VARCHAR ( 255 ) BEGIN
DECLARE
chars_str VARCHAR ( 100 ) DEFAULT 'abcdefghijklmnopqrstuvwxyzABCDEFJHIJKLMNOPQRSTUVWXYZ';
DECLARE
return_str VARCHAR ( 255 ) DEFAULT '';
DECLARE
i INT DEFAULT 0;
WHILE
i < n DO
SET return_str = CONCAT(
return_str,
SUBSTRING( chars_str, FLOOR( 1+RAND ()* 52 ), 1 ));
SET i = i + 1;
END WHILE;
RETURN return_str;
END //
DELIMITER;#假如要删除
#drop function rand_string;
随机产生班级编号
#用于随机产生多少到多少的编号
DELIMITER //
CREATE FUNCTION rand_num ( from_num INT, to_num INT ) RETURNS INT ( 11 ) BEGIN
DECLARE
i INT DEFAULT 0;
SET i = FLOOR(
from_num + RAND()*(
to_num - from_num + 1
));
RETURN i;
END //
DELIMITER;#假如要删除
#drop function rand_num;
创建存储过程
#创建往stu表中插入数据的存储过程
DELIMITER //
CREATE PROCEDURE insert_stu ( START INT, max_num INT ) BEGIN
DECLARE
i INT DEFAULT 0;
SET autocommit = 0;#设置手动提交事务
REPEAT#循环
SET i = i + 1;#赋值
INSERT INTO student ( stuno, NAME, age, classId )
VALUES
((
START + i
),
rand_string ( 6 ),
rand_num ( 1, 50 ),
rand_num ( 1, 1000 ));
UNTIL i = max_num
END REPEAT;
COMMIT;#提交事务
END //
DELIMITER;#假如要删除
#drop PROCEDURE insert_stu;
创建往class表中插入数据的存储过程
#执行存储过程,往class表添加随机数据
DELIMITER //
CREATE PROCEDURE `insert_class` ( max_num INT ) BEGIN
DECLARE
i INT DEFAULT 0;
SET autocommit = 0;
REPEAT
SET i = i + 1;
INSERT INTO class ( classname, address, monitor )
VALUES
(
rand_string ( 8 ),
rand_string ( 10 ),
rand_num ( 1, 100000 ));
UNTIL i = max_num
END REPEAT;
COMMIT;
END //
DELIMITER;#假如要删除
#drop PROCEDURE insert_class;
调用存储过程
class
#执行存储过程,往class表添加1万条数据
CALL insert_class(10000);
stu
#执行存储过程,往stu表添加50万条数据
CALL insert_stu(100000,500000);
删除某表上的索引
创建存储过程
DELIMITER //
CREATE PROCEDURE `proc_drop_index` (
dbname VARCHAR ( 200 ),
tablename VARCHAR ( 200 )) BEGIN
DECLARE
done INT DEFAULT 0;
DECLARE
ct INT DEFAULT 0;
DECLARE
_index VARCHAR ( 200 ) DEFAULT '';
DECLARE
_cur CURSOR FOR SELECT
index_name
FROM
information_schema.STATISTICS
WHERE
table_schema = dbname
AND table_name = tablename
AND seq_in_index = 1
AND index_name <> 'PRIMARY';#每个游标必须使用不同的declare continue handler for not found set done=1来控制游标的结束
DECLARE
CONTINUE HANDLER FOR NOT FOUND
SET done = 2;#若没有数据返回,程序继续,并将变量done设为2
OPEN _cur;
FETCH _cur INTO _index;
WHILE
_index <> '' DO
SET @str = CONCAT( "drop index ", _index, " on ", tablename );
PREPARE sql_str
FROM
@str;
EXECUTE sql_str;
DEALLOCATE PREPARE sql_str;
SET _index = '';
FETCH _cur INTO _index;
END WHILE;
CLOSE _cur;
END //
DELIMITER;
执行存储过程
CALL proc_drop_index("dbname","tablename");
拓展:Alibaba《Java开发手册》
索引文件具有 B-Tree 的最左前缀匹配特性,如果左边的值未确定,那么无法使用此索引。
如果此时再插入一条主键值为9 的记录,那它插入的位置就如下图:
可这个数据页已经满了,再插进来咋办呢?我们需要把当前页面分裂成两个页面,把本页中的一些记录移动到新创建的这个页中。页面分裂和记录移位意味着什么?意味着: 性能损耗!所以如果我们想尽量避免这样无谓的性能损耗,最好让插入的记录的主键值依次递增,这样就不会发生这样的性能损耗了。所以我们建议:让主键具有AUTO_INCREMENT ,让存储引擎自己为表生成主键,而不是我们手动插入 ,比如: person_info 表:
CREATE TABLE person_info (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
NAME VARCHAR ( 100 ) NOT NULL,
birthday DATE NOT NULL,
phone_number CHAR ( 11 ) NOT NULL,
country VARCHAR ( 100 ) NOT NULL,
PRIMARY KEY ( id ),
KEY idx_name_birthday_phone_number ( NAME ( 10 ), birthday, phone_number )
);
我们自定义的主键列id 拥有AUTO_INCREMENT
属性,在插入记录时存储引擎会自动为我们填入自增的主键值。这样的主键占用空间小,顺序写入,减少页分裂。
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE student.name LIKE 'abc%';
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE LEFT(student.name,3) = 'abc';
创建索引
CREATE INDEX idx_name ON student(NAME);
第一种:索引优化生效
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE student.name LIKE 'abc%';
第二种:索引优化失效
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE LEFT(student.name,3) = 'abc';
+----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 498917 | 100.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 2 warnings (0.00 sec)
mysql> SELECT SQL_NO_CACHE * FROM student WHERE LEFT(student.name,3) = 'abc';
+--------+--------+--------+------+---------+
| id | stuno | name | age | classId |
+--------+--------+--------+------+---------+
| 399 | 100399 | ABcKtL | 24 | 198 |
| 16470 | 116470 | ABcJlg | 47 | 251 |
| 27952 | 127952 | ABcJmj | 10 | 397 |
| 54809 | 154809 | aBClvu | 37 | 495 |
| 61540 | 161540 | abclUS | 30 | 374 |
| 83160 | 183160 | aBCjpV | 34 | 593 |
| 89664 | 189664 | aBCjmJ | 34 | 350 |
| 240498 | 340498 | aBCksj | 41 | 491 |
| 245214 | 345214 | abciJU | 23 | 568 |
| 258459 | 358459 | aBClxC | 23 | 566 |
| 300169 | 400169 | aBClxC | 21 | 412 |
| 300328 | 400328 | ABcJnn | 27 | 870 |
| 324684 | 424684 | aBCkrg | 30 | 566 |
| 416907 | 516907 | ABcHgI | 46 | 607 |
| 424459 | 524459 | abclVU | 39 | 192 |
| 445547 | 545547 | ABcJpw | 16 | 180 |
| 454772 | 554772 | AbCHFf | 37 | 313 |
| 466466 | 566466 | abckRF | 26 | 725 |
| 475708 | 575708 | abclWY | 4 | 415 |
| 486611 | 586611 | ABcLwb | 41 | 948 |
| 490152 | 590152 | ABcHfC | 24 | 717 |
+--------+--------+--------+------+---------+
21 rows in set, 1 warning (0.12 sec)
type为“ALL”,表示没有使用到索引
下列哪个sql语句可以用到索引。(假设name字段上设置有索引)
# 未使用到索引
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE name=123;
+----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | student | NULL | ALL | idx_name | NULL | NULL | NULL | 498917 | 10.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 4 warnings (0.00 sec)
# 使用到索引
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE name='123';
+----+-------------+---------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | student | NULL | ref | idx_name | idx_name | 63 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 2 warnings (0.00 sec)
ALTER TABLE student DROP INDEX idx_name;
ALTER TABLE student DROP INDEX idx_age;
ALTER TABLE student DROP INDEX idx_age_classid;
EXPLAIN SELECT SQL_NO_CACHE * FROM student
WHERE student.age=30 AND student.classId>20 AND student.name = 'abc' ;
create index idx_age_name_classid on student(age,name,classid);
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE student.age=30 AND student.name =
-> 'abc' AND student.classId>20 ;
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | student | NULL | range | idx_age_name_classid | idx_age_name_classid | 73 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
1 row in set, 2 warnings (0.00 sec)
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age IS NULL;
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age IS NOT NULL;
拓展:Alibaba《Java开发手册》
【强制】页面搜索严禁左模糊或者全模糊,如果需要请走搜索引擎来解决。
# 未使用到索引
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age = 10 OR classid = 100;
+----+-------------+---------+------------+------+----------------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | student | NULL | ALL | idx_age_name_classid | NULL | NULL | NULL | 498917 | 11.88 | Using where |
+----+-------------+---------+------------+------+----------------------+------+---------+------+--------+----------+-------------+
1 row in set, 2 warnings (0.00 sec)
# 使用到索引
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age = 10 OR name = 'Abel';
+----+-------------+---------+------------+------+----------------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | student | NULL | ALL | idx_age_name_classid | NULL | NULL | NULL | 498917 | 11.88 | Using where |
+----+-------------+---------+------------+------+----------------------+------+---------+------+--------+----------+-------------+
1 row in set, 2 warnings (0.00 sec)
统一使用utf8mb4( 5.5.3版本以上支持)兼容性更好,统一字符集可以避免由于字符集转换产生的乱码。不同的字符集进行比较前需要进行转换会造成索引失效。
CREATE TABLE IF NOT EXISTS `type` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY (`id`)
);
#图书
CREATE TABLE IF NOT EXISTS `book` (
`bookid` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY (`bookid`)
);
#向分类表中添加20条记录
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
#向图书表中添加20条记录
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO book(card) VALUES(FLOOR(1 + (RAND() * 20)));
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM `type` LEFT JOIN book ON type.card = book.card;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | type | NULL | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | NULL |
| 1 | SIMPLE | book | NULL | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 2 warnings (0.00 sec)
结论:type 有All
添加索引优化
ALTER TABLE book ADD INDEX Y ( card); #【被驱动表】,可以避免全表扫描
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM `type` LEFT JOIN book ON type.card = book.card;
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
| 1 | SIMPLE | type | NULL | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | NULL |
| 1 | SIMPLE | book | NULL | ref | Y | Y | 4 | atguigudb2.type.card | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
可以看到第二行的 type 变为了 ref,rows 也变成了优化比较明显。这是由左连接特性决定的。LEFT JOIN
条件用于确定如何从右表搜索行,左边一定都有,所以右边是我们的关键点,一定需要建立索引。
ALTER TABLE `type` ADD INDEX X (card); #【驱动表】,无法避免全表扫描
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM `type` LEFT JOIN book ON type.card = book.card;
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
| 1 | SIMPLE | type | NULL | index | NULL | X | 4 | NULL | 20 | 100.00 | Using index |
| 1 | SIMPLE | book | NULL | ref | Y | Y | 4 | atguigudb2.type.card | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
DROP INDEX Y ON book;
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM `type` LEFT JOIN book ON type.card = book.card;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | type | NULL | index | NULL | X | 4 | NULL | 20 | 100.00 | Using index |
| 1 | SIMPLE | book | NULL | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 2 warnings (0.00 sec)
drop index X on type;
drop index Y on book;
换成 inner join(MySQL自动选择驱动表)
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM type INNER JOIN book ON type.card=book.card;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | type | NULL | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | NULL |
| 1 | SIMPLE | book | NULL | ALL | NULL | NULL | NULL | NULL | 20 | 10.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 2 warnings (0.00 sec)
添加索引优化
ALTER TABLE book ADD INDEX Y ( card);
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM type INNER JOIN book ON type.card=book.card;
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
| 1 | SIMPLE | type | NULL | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | NULL |
| 1 | SIMPLE | book | NULL | ref | Y | Y | 4 | atguigudb2.type.card | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
ALTER TABLE type ADD INDEX X (card);
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM type INNER JOIN book ON type.card=book.card;
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
| 1 | SIMPLE | type | NULL | index | X | X | 4 | NULL | 20 | 100.00 | Using index |
| 1 | SIMPLE | book | NULL | ref | Y | Y | 4 | atguigudb2.type.card | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
接着:
DROP INDEX X ON `type`;
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM TYPE INNER JOIN book ON type.card=book.card;
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
| 1 | SIMPLE | TYPE | NULL | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | NULL |
| 1 | SIMPLE | book | NULL | ref | Y | Y | 4 | atguigudb2.TYPE.card | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+------+---------+----------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
接着:
ALTER TABLE `type` ADD INDEX X (card);
mysql> EXPLAIN SELECT SQL_NO_CACHE * FROM `type` INNER JOIN book ON type.card=book.card;
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
| 1 | SIMPLE | type | NULL | index | X | X | 4 | NULL | 20 | 100.00 | Using index |
| 1 | SIMPLE | book | NULL | ref | Y | Y | 4 | atguigudb2.type.card | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+----------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
Index Nested-Loop Join
EXPLAIN SELECT * FROM t1 STRAIGHT_JOIN t2 ON (t1.a=t2.a);
如果直接使用join语句,MySQL优化器可能会选择表t1或t2作为驱动表,这样会影响我们分析SQL语句的执行过程。所以,为了便于分析执行过程中的性能问题,我改用straight_join
让MySQL使用固定的连接方式执行查询,这样优化器只会按照我们指定的方式去join。在这个语句里,t1 是驱动表,t2是被驱动表。
可以看到,在这条语句里,被驱动表t2的字段a上有索引,join过程用上了这个索引,因此这个语句的执行流程是这样的:
这个过程是先遍历表t1,然后根据从表t1中取出的每行数据中的a值,去表t2中查找满足条件的记录。在形式上,这个过程就跟我们写程序时的嵌套查询类似,并且可以用上被驱动表的索引,所以我们称之为“Index Nested-Loop Join”,简称NLJ。
它对应的流程图如下所示:
在这个流程里:
两个结论:
- 使用join语句,性能比强行拆成多个单表执行SQL语句的性能要好;
- 如果使用join语句的话,需要让小表做驱动表。
Simple Nested-Loop Join
Block Nested-Loop Join
执行流程图也就变成这样:
在决定哪个表做驱动表的时候,应该是两个表按照各自的条件过滤,过滤完成之后,计算参与join的各个字段的总数据量,数据量小的那个表,就是“小表”,应该作为驱动表。
总结
大表作为被驱动表
。减少外层循环的次数。小结果集的表选为驱动表
。选择相信MySQL优化策略。MySQL从4.1版本开始支持子查询,使用子查询可以进行SELECT语句的嵌套查询,即一个SELECT查询的结果作为另一个SELECT语句的条件。子查询可以一次性完成很多逻辑上需要多个步骤才能完成的SQL操作。
子查询是 MySQL 的一项重要的功能,可以帮助我们通过一个 SQL 语句实现比较复杂的查询。但是,子查询的执行效率不高
① 执行子查询时,MySQL需要为内层查询语句的查询结果建立一个临时表,然后外层查询语句从临时表中查询记录。查询完毕后,再撤销这些临时表。这样会消耗过多的CPU和IO资源,产生大量的慢查询。
② 子查询的结果集存储的临时表,不论是内存临时表还是磁盘临时表都不会存在索引,所以查询性能会受到一定的影响。
③ 对于返回结果集比较大的子查询,其对查询性能的影响也就越大。在MySQL中,可以使用连接(JOIN)查询来替代子查询。连接查询不需要建立临时表,其速度比子查询要快,如果查询中使用索引的话,性能就会更好。
结论:尽量不要使用NOT IN 或者 NOT EXISTS,用LEFT JOIN xxx ON xx WHERE xx IS NULL替代