mysql索引优化2

mysql 索引分析案例2_两表优化

建表
案例1分析
案例1 结论

建表

mysql> create table class(
    -> id int(10) not null auto_increment,
    -> card int(10) not null,
    -> primary key(id)
    -> );
Query OK, 0 rows affected (0.21 sec)

mysql> create table book(
    -> bookid int(10) not null auto_increment,
    -> card int(10) not null,
    -> primary key(bookid)
    -> );
Query OK, 0 rows affected (0.18 sec)

//插入数据
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(1+(rand()*20));
insert into class(card) values(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)));
insert into book(card) values(floor(1+(rand()*20)))


//查看数据
mysql> select * from book;
+--------+------+
| bookid | card |
+--------+------+
|      1 |    7 |
|      2 |    7 |
|      3 |   12 |
|      4 |   19 |
|      5 |    1 |
|      6 |    5 |
|      7 |    3 |
|      8 |   20 |
|      9 |   11 |
|     10 |   16 |
|     11 |    4 |
|     12 |   12 |
|     13 |   10 |
|     14 |   10 |
|     15 |    1 |
|     16 |   14 |
|     17 |    6 |
|     18 |    6 |
|     19 |   14 |
|     20 |   11 |
+--------+------+
20 rows in set (0.00 sec)

mysql> select * from class;
+----+------+
| id | card |
+----+------+
|  1 |   15 |
|  2 |   10 |
|  3 |    3 |
|  4 |    2 |
|  5 |    3 |
|  6 |    5 |
|  7 |   15 |
|  8 |   20 |
|  9 |   15 |
| 10 |   14 |
| 11 |    4 |
| 12 |   16 |
| 13 |    7 |
| 14 |    4 |
| 15 |    2 |
| 16 |   14 |
| 17 |    3 |
| 18 |   12 |
| 19 |   11 |
| 20 |   16 |
+----+------+
20 rows in set (0.00 sec)

案例1分析

两张表数据没有任何实际意义,案例也没有任何意义,只是为了出问题,解决问题关于案例并不由纠结太多- - #
1.使用left join 查询两张表

mysql> explain select * from class left join book on class.card=book.card;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | class | ALL  | NULL          | NULL | NULL    | NULL |   20 | NULL                                               |
|  1 | SIMPLE      | book  | ALL  | NULL          | NULL | NULL    | NULL |   20 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
2 rows in set (0.00 sec)

我们发现出现了全表扫描,先来解决这个问题,但是涉及到了两张表我们应该在那张表来建立索引,大胆假设,小心求证我们一一来尝试一下就知道了首先我们选择book的card来建立索引

mysql> create index idx_book_c on book(card);
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

我们在尝试一下使用左连接查询在右表建立索引

mysql> explain select * from class left join book on class.card=book.card;
+----+-------------+-------+------+---------------+------------+---------+-----------------+------+-------------+
| id | select_type | table | type | possible_keys | key        | key_len | ref             | rows | Extra       |
+----+-------------+-------+------+---------------+------------+---------+-----------------+------+-------------+
|  1 | SIMPLE      | class | ALL  | NULL          | NULL       | NULL    | NULL            |   20 | NULL        |
|  1 | SIMPLE      | book  | ref  | idx_book_c    | idx_book_c | 4       | test.class.card |    1 | Using index |
+----+-------------+-------+------+---------------+------------+---------+-----------------+------+-------------+
2 rows in set (0.00 sec)

我们发现book全表扫描已经变成了ref 也使用到了索引,且查询行数变成了1 已经有明显的改变,我们在尝试一下在左表建立索引对比一下

//删除索引
drop index idx_book_c on book;

//在左表建立索引
create index idx_class_c on class(card);

//再次尝试
mysql> explain select * from class left join book on class.card=book.card;
+----+-------------+-------+-------+---------------+-------------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type  | possible_keys | key         | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | class | index | NULL          | idx_class_c | 4       | NULL |   20 | Using index                                        |
|  1 | SIMPLE      | book  | ALL   | NULL          | NULL        | NULL    | NULL |   20 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+----------------------------------------------------+
2 rows in set (0.00 sec)

我们发现type 为index 且 row20 并没有得到优化

案例1 结论

由于是左连接,left join 条件用于确定如何草右表搜索行,左边一定都有,所以右边使我们的关键点,一定需要建立索引.
索引我们还是删除之前的索引

mysql> drop index idx_class_c on class;
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

//还是在右表建立索引吧
mysql> create index idx_book_c on book(card);
Query OK, 0 rows affected (0.20 sec)
Records: 0  Duplicates: 0  Warnings: 0

//发现回到了刚开始的样子	- - !
mysql> explain select * from class left join book on class.card=book.card;
+----+-------------+-------+------+---------------+------------+---------+-----------------+------+-------------+
| id | select_type | table | type | possible_keys | key        | key_len | ref             | rows | Extra       |
+----+-------------+-------+------+---------------+------------+---------+-----------------+------+-------------+
|  1 | SIMPLE      | class | ALL  | NULL          | NULL       | NULL    | NULL            |   20 | NULL        |
|  1 | SIMPLE      | book  | ref  | idx_book_c    | idx_book_c | 4       | test.class.card |    1 | Using index |
+----+-------------+-------+------+---------------+------------+---------+-----------------+------+-------------+
2 rows in set (0.00 sec)

##案例2
不想多说了同理 ,如果采用右连接当然是左表建立索引比较好

mysql 索引分析案例2_两表优化

建表

案例1分析

案例1 结论

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