提高性能的几条sql语句

mysql版本：

mysql> select version();
+------------+
| version()  |
+------------+
| 5.6.44-log |
+------------+
1 row in set (0.00 sec)

一、尽量不使用select * from查出所有列

有时候一个表字段太多，有的字段内容还很大，查询用到的列，节省资源、减少网络开销。

为什么大家都说SELECT * 效率低

---

二、避免使用or进行查询

其中sname和age字段均设置成为了索引

• 使用or进行查询的时候（未命中索引）：

mysql> explain select * from student where sname="李磊" or age=20;
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | student | ALL  | sname,age     | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

• 可以使用union进行查询（命中了索引）：

mysql> explain select * from student where sname="李磊" union select * from student where age=20;
+----+--------------+------------+------+---------------+-------+---------+-------+------+-----------------------+
| id | select_type  | table      | type | possible_keys | key   | key_len | ref   | rows | Extra                 |
+----+--------------+------------+------+---------------+-------+---------+-------+------+-----------------------+
|  1 | PRIMARY      | student    | ref  | sname         | sname | 62      | const |    1 | Using index condition |
|  2 | UNION        | student    | ref  | age           | age   | 4       | const |    1 | NULL                  |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL  | NULL    | NULL  | NULL | Using temporary       |
+----+--------------+------------+------+---------------+-------+---------+-------+------+-----------------------+
3 rows in set (0.00 sec)

---

三、如果被查询的列是字符串，查询条件则用引号括起来

其中sname为索引列

3.1 当sname字段类型为varchar，不用双引号查询：

mysql> explain select * from student where sname=666;
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | student | ALL  | sname         | NULL | NULL    | NULL |    6 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

没用到索引，进行了全表扫描。

3.2 当sname字段类型为varchar，用双引号查询：

mysql> explain select * from student where sname="666";
+----+-------------+---------+------+---------------+-------+---------+-------+------+-----------------------+
| id | select_type | table   | type | possible_keys | key   | key_len | ref   | rows | Extra                 |
+----+-------------+---------+------+---------------+-------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | student | ref  | sname         | sname | 62      | const |    1 | Using index condition |
+----+-------------+---------+------+---------------+-------+---------+-------+------+-----------------------+
1 row in set (0.00 sec

3.3 当你把sname字段类型更换为int、tinyint等这种数字类型的时候：

mysql> explain select * from student where sname=666;
+----+-------------+---------+------+---------------+-------+---------+-------+------+-------+
| id | select_type | table   | type | possible_keys | key   | key_len | ref   | rows | Extra |
+----+-------------+---------+------+---------------+-------+---------+-------+------+-------+
|  1 | SIMPLE      | student | ref  | sname         | sname | 4       | const |    1 | NULL  |
+----+-------------+---------+------+---------------+-------+---------+-------+------+-------+
1 row in set (0.00 sec)

就又用到了索引。

---

四、建立联合索引要遵循最左侧匹配原则

其中sname、age、cid依次顺序为联合索引

4.1 全值匹配查询时

mysql> explain select sname from student where sname="李磊" and age=20 and cid=1;
+----+-------------+---------+------+---------------+-------------+---------+-------------------+------+--------------------------+
| id | select_type | table   | type | possible_keys | key         | key_len | ref               | rows | Extra                    |
+----+-------------+---------+------+---------------+-------------+---------+-------------------+------+--------------------------+
|  1 | SIMPLE      | student | ref  | snameagecid   | snameagecid | 70      | const,const,const |    1 | Using where; Using index |
+----+-------------+---------+------+---------------+-------------+---------+-------------------+------+--------------------------+
1 row in set (0.00 sec)

不管sname、age、cid三列在哪个位置，只要三者都有，Mysql中有查询优化器，会自动优化查询顺序，都能够使用索引。
关于mysql最左侧匹配原则

4.2 匹配左边的列时

mysql> explain select sname from student where sname="李磊";
+----+-------------+---------+------+---------------+-------------+---------+-------+------+--------------------------+
| id | select_type | table   | type | possible_keys | key         | key_len | ref   | rows | Extra                    |
+----+-------------+---------+------+---------------+-------------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | student | ref  | snameagecid   | snameagecid | 62      | const |    1 | Using where; Using index |
+----+-------------+---------+------+---------------+-------------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select sname from student where sname="李磊" and age=20;
+----+-------------+---------+------+---------------+-------------+---------+-------------+------+--------------------------+
| id | select_type | table   | type | possible_keys | key         | key_len | ref         | rows | Extra                    |
+----+-------------+---------+------+---------------+-------------+---------+-------------+------+--------------------------+
|  1 | SIMPLE      | student | ref  | snameagecid   | snameagecid | 66      | const,const |    1 | Using where; Using index |
+----+-------------+---------+------+---------------+-------------+---------+-------------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select sname from student where sname="李磊" and age=20 and cid=1;
+----+-------------+---------+------+---------------+-------------+---------+-------------------+------+--------------------------+
| id | select_type | table   | type | possible_keys | key         | key_len | ref               | rows | Extra                    |
+----+-------------+---------+------+---------------+-------------+---------+-------------------+------+--------------------------+
|  1 | SIMPLE      | student | ref  | snameagecid   | snameagecid | 70      | const,const,const |    1 | Using where; Using index |
+----+-------------+---------+------+---------------+-------------+---------+-------------------+------+--------------------------+
1 row in set (0.00 sec)

遵循mysql最左侧匹配原则，也都使用上了索引。

4.3 既不是全值匹配，又不从最左侧开始

mysql> explain select sname from student where cid=1;
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
| id | select_type | table   | type  | possible_keys | key         | key_len | ref  | rows | Extra                    |
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | student | index | NULL          | snameagecid | 70      | NULL |    6 | Using where; Using index |
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select sname from student where age=20;
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
| id | select_type | table   | type  | possible_keys | key         | key_len | ref  | rows | Extra                    |
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | student | index | NULL          | snameagecid | 70      | NULL |    6 | Using where; Using index |
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select sname from student where age=20 and cid=1;
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
| id | select_type | table   | type  | possible_keys | key         | key_len | ref  | rows | Extra                    |
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | student | index | NULL          | snameagecid | 70      | NULL |    6 | Using where; Using index |
+----+-------------+---------+-------+---------------+-------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

type为index：只不过它的扫描顺序是按照索引的顺序。这种扫描根据索引然后回表取数据，和all相比，他们都是取得了全表的数据，而且index要先读索引而且要回表随机取数据。

---

五、避免在索引列进行计算

其中，age字段为索引

mysql> explain select * from student where age-1=19;
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | student | ALL  | NULL          | NULL | NULL    | NULL |    6 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

索引失效，全盘扫描。

---

六、使用like查询的时候，%在前面不走索引

其中sname字段为索引

mysql> select * from student;
+----+-----------+-----+-----+
| id | sname     | age | cid |
+----+-----------+-----+-----+
|  1 | 李磊      |  20 |   1 |
|  2 | 李关亮    |  21 |   2 |
|  3 | 李腾      |  21 |   3 |
|  4 | 黄成志    |  23 |   4 |
|  5 | 张国栋    |  21 |   2 |
+----+-----------+-----+-----+
5 rows in set (0.00 sec)

• 当%在前面的时候（全表扫描）：

mysql> explain select * from student where sname like '%李';
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | student | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

• 当%在后面的时候（使用了索引）：

mysql> explain select * from student where sname like '磊%';
+----+-------------+---------+-------+---------------+-------+---------+------+------+-----------------------+
| id | select_type | table   | type  | possible_keys | key   | key_len | ref  | rows | Extra                 |
+----+-------------+---------+-------+---------------+-------+---------+------+------+-----------------------+
|  1 | SIMPLE      | student | range | sname         | sname | 62      | NULL |    1 | Using index condition |
+----+-------------+---------+-------+---------------+-------+---------+------+------+-----------------------+
1 row in set (0.01 sec)

• %在两边的时候（全表扫描）：

mysql> explain select * from student where sname like "%国%";
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | student | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

如果需求是必须要百分号在前面，可参考：MySQL模糊查询用法大全（正则、通配符、内置函数等）

七、limit分页的优化

这个表存了1千多万条数据：

mysql> select count(*) from fa_ceshi;
+----------+
| count(*) |
+----------+
| 11651681 |
+----------+
1 row in set (2.43 sec)

当看第1000000页时，查询效率很低下：

mysql> select * from fa_ceshi limit 1000000,20;
+---------+---------------------+
| id      | times               |
+---------+---------------------+
| 1000001 | 2020-05-10 00:04:20 |
| 1000002 | 2020-05-10 00:04:20 |
| 1000003 | 2020-05-10 00:04:21 |
| 1000004 | 2020-05-10 00:04:21 |
| 1000005 | 2020-05-10 00:04:21 |
| 1000006 | 2020-05-10 00:04:21 |
| 1000007 | 2020-05-10 00:04:21 |
| 1000008 | 2020-05-10 00:04:21 |
| 1000009 | 2020-05-10 00:04:21 |
| 1000010 | 2020-05-10 00:04:21 |
| 1000011 | 2020-05-10 00:04:21 |
| 1000012 | 2020-05-10 00:04:22 |
| 1000013 | 2020-05-10 00:04:22 |
| 1000014 | 2020-05-10 00:04:22 |
| 1000015 | 2020-05-10 00:04:22 |
| 1000016 | 2020-05-10 00:04:22 |
| 1000017 | 2020-05-10 00:04:22 |
| 1000018 | 2020-05-10 00:04:22 |
| 1000019 | 2020-05-10 00:04:22 |
| 1000020 | 2020-05-10 00:04:22 |
+---------+---------------------+
20 rows in set (0.26 sec)

这时候mysql并不是跳过1000000偏移量去直接取后面的数据，而是需要查询1000000条数据然后只返回最后的20条，前面1000000条将会抛弃，那自然而然分页越往后效率越低。

《高性能mysql》：如果所有的页面被访问的频率都相同，那么这样的查询平均需要访问半个表的数据。要优化这种查询，要么是在页面限制分页数量，要么是优化大偏移量的性能。

• 优化方案一：

mysql> select * from fa_ceshi where id>1000000 limit 20;
+---------+---------------------+
| id      | times               |
+---------+---------------------+
| 1000001 | 2020-05-10 00:04:20 |
| 1000002 | 2020-05-10 00:04:20 |
| 1000003 | 2020-05-10 00:04:21 |
| 1000004 | 2020-05-10 00:04:21 |
| 1000005 | 2020-05-10 00:04:21 |
| 1000006 | 2020-05-10 00:04:21 |
| 1000007 | 2020-05-10 00:04:21 |
| 1000008 | 2020-05-10 00:04:21 |
| 1000009 | 2020-05-10 00:04:21 |
| 1000010 | 2020-05-10 00:04:21 |
| 1000011 | 2020-05-10 00:04:21 |
| 1000012 | 2020-05-10 00:04:22 |
| 1000013 | 2020-05-10 00:04:22 |
| 1000014 | 2020-05-10 00:04:22 |
| 1000015 | 2020-05-10 00:04:22 |
| 1000016 | 2020-05-10 00:04:22 |
| 1000017 | 2020-05-10 00:04:22 |
| 1000018 | 2020-05-10 00:04:22 |
| 1000019 | 2020-05-10 00:04:22 |
| 1000020 | 2020-05-10 00:04:22 |
+---------+---------------------+
20 rows in set (0.00 sec)

用id返回最大查询记录（偏移量），这样可以跳过偏移量，从而提升不少效率。

• 优化方案二（其中id为索引）：

mysql> select * from fa_ceshi where id>1000000 order by id limit 20;
+---------+---------------------+
| id      | times               |
+---------+---------------------+
| 1000001 | 2020-05-10 00:04:20 |
| 1000002 | 2020-05-10 00:04:20 |
| 1000003 | 2020-05-10 00:04:21 |
| 1000004 | 2020-05-10 00:04:21 |
| 1000005 | 2020-05-10 00:04:21 |
| 1000006 | 2020-05-10 00:04:21 |
| 1000007 | 2020-05-10 00:04:21 |
| 1000008 | 2020-05-10 00:04:21 |
| 1000009 | 2020-05-10 00:04:21 |
| 1000010 | 2020-05-10 00:04:21 |
| 1000011 | 2020-05-10 00:04:21 |
| 1000012 | 2020-05-10 00:04:22 |
| 1000013 | 2020-05-10 00:04:22 |
| 1000014 | 2020-05-10 00:04:22 |
| 1000015 | 2020-05-10 00:04:22 |
| 1000016 | 2020-05-10 00:04:22 |
| 1000017 | 2020-05-10 00:04:22 |
| 1000018 | 2020-05-10 00:04:22 |
| 1000019 | 2020-05-10 00:04:22 |
| 1000020 | 2020-05-10 00:04:22 |
+---------+---------------------+
20 rows in set (0.00 sec)
mysql> explain select * from fa_ceshi where id>1000000 order by id limit 20;
+----+-------------+----------+-------+---------------+---------+---------+------+---------+-------------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref  | rows    | Extra       |
+----+-------------+----------+-------+---------------+---------+---------+------+---------+-------------+
|  1 | SIMPLE      | fa_ceshi | range | PRIMARY,id    | PRIMARY | 4       | NULL | 5138205 | Using where |
+----+-------------+----------+-------+---------------+---------+---------+------+---------+-------------+
1 row in set (0.00 sec)

用id返回最大查询记录（偏移量），跳过偏移量，在配合order by+索引，提高查询效率。

• 优化方案三（其中id为索引）：

mysql> select * from fa_ceshi where id BETWEEN 10000000 AND 10000020 order by id;
+----------+---------------------+
| id       | times               |
+----------+---------------------+
| 10000000 | 2020-05-21 15:16:50 |
| 10000001 | 2020-05-21 15:16:50 |
| 10000002 | 2020-05-21 15:16:50 |
| 10000003 | 2020-05-21 15:16:50 |
| 10000004 | 2020-05-21 15:16:50 |
| 10000005 | 2020-05-21 15:16:50 |
| 10000006 | 2020-05-21 15:16:50 |
| 10000007 | 2020-05-21 15:16:50 |
| 10000008 | 2020-05-21 15:16:50 |
| 10000009 | 2020-05-21 15:16:51 |
| 10000010 | 2020-05-21 15:16:51 |
| 10000011 | 2020-05-21 15:16:51 |
| 10000012 | 2020-05-21 15:16:51 |
| 10000013 | 2020-05-21 15:16:51 |
| 10000014 | 2020-05-21 15:16:51 |
| 10000015 | 2020-05-21 15:16:51 |
| 10000016 | 2020-05-21 15:16:51 |
| 10000017 | 2020-05-21 15:16:51 |
| 10000018 | 2020-05-21 15:16:52 |
| 10000019 | 2020-05-21 15:16:52 |
| 10000020 | 2020-05-21 15:16:52 |
+----------+---------------------+
21 rows in set (0.00 sec)

mysql> explain select * from fa_ceshi where id BETWEEN 10000000 AND 10000020 order by id;
+----+-------------+----------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+----------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | fa_ceshi | range | PRIMARY,id    | PRIMARY | 4       | NULL |   21 | Using where |
+----+-------------+----------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

《高性能mysql》：有时候也可以将limit转化为已知位置的查询，让Mysql通过范围扫描获得到对应的结果，例如，id为索引，并且预先列出了边界值。

• 优化方案四：

基本需求用户是查不到这么多页的，如果产品经理硬要让你查那么多页，这和让你根据手机壳改变颜色有什么区别（貌似有人实现了）？那就干一架吧！

八、判断是否为NULL用is

先看下表内数据：

mysql> select * from student;
+----+-----------+-------+-----+---------------------+
| id | sname     | age   | cid | times               |
+----+-----------+-------+-----+---------------------+
|  2 | 李关亮    | 21.00 |   2 | 2020-07-16 18:56:03 |
|  3 | 李腾      | 21.00 |   3 | 2020-07-16 18:56:07 |
|  4 | 黄成志    | 23.00 |   4 | 2020-07-29 18:56:11 |
|  5 | 张国栋    | 21.00 |   2 | 2020-07-31 18:56:15 |
|  8 | NULL      | 54.00 |   1 | 2020-07-02 20:54:03 |
+----+-----------+-------+-----+---------------------+
5 rows in set (0.00 sec)

• 用=号查询：

mysql> select * from student where sname=null;
Empty set (0.00 sec)

• 用is null查询：

mysql> select * from student where sname is null;
+----+-------+-------+-----+---------------------+
| id | sname | age   | cid | times               |
+----+-------+-------+-----+---------------------+
|  8 | NULL  | 54.00 |   1 | 2020-07-02 20:54:03 |
+----+-------+-------+-----+---------------------+
1 row in set (0.00 sec)

• 查询!=李关亮的所有数据：

mysql> select * from student where sname!="李关亮";
+----+-----------+-------+-----+---------------------+
| id | sname     | age   | cid | times               |
+----+-----------+-------+-----+---------------------+
|  3 | 李腾      | 21.00 |   3 | 2020-07-16 18:56:07 |
|  4 | 黄成志    | 23.00 |   4 | 2020-07-29 18:56:11 |
|  5 | 张国栋    | 21.00 |   2 | 2020-07-31 18:56:15 |
+----+-----------+-------+-----+---------------------+
3 rows in set (0.00 sec)

发现那条空数据没有查出。

mysql> select * from student where sname!="李关亮" or sname is null;
+----+-----------+-------+-----+---------------------+
| id | sname     | age   | cid | times               |
+----+-----------+-------+-----+---------------------+
|  3 | 李腾      | 21.00 |   3 | 2020-07-16 18:56:07 |
|  4 | 黄成志    | 23.00 |   4 | 2020-07-29 18:56:11 |
|  5 | 张国栋    | 21.00 |   2 | 2020-07-31 18:56:15 |
|  8 | NULL      | 54.00 |   1 | 2020-07-02 20:54:03 |
+----+-----------+-------+-----+---------------------+
4 rows in set (0.00 sec)

再加个条件，或者sname is null即可查出。

mysql> explain select * from student where sname is not null;
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | student | ALL  | sname         | NULL | NULL    | NULL |    4 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

is not null导致索引失效。

以上实例都是null引起的一系列问题，最根本的解决方案禁止设置字段为null，给个默认值也可

九、反向查询导致索引失效

其中id为唯一索引

mysql> explain select * from student where id!=2;
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | student | ALL  | PRIMARY,id    | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

不等于导致索引失效，全表扫描

其中cid为单列索引

mysql> explain select * from student where cid<>2;
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | student | ALL  | cid           | NULL | NULL    | NULL |    4 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

不等于导致索引失效，全表扫描。

十、 在只取一条并已知查询会有重复数据的情况下用limit1提高查询效率

先看下表内数据量：

mysql> select count(*) from fa_ceshi;
+----------+
| count(*) |
+----------+
| 11651681 |
+----------+
1 row in set (2.08 sec)

查询某个时间点（耗时挺长）：

mysql> select * from fa_ceshi where times="2020-05-15 04:09:24";
+---------+---------------------+
| id      | times               |
+---------+---------------------+
| 4999657 | 2020-05-15 04:09:24 |
| 4999658 | 2020-05-15 04:09:24 |
| 4999659 | 2020-05-15 04:09:24 |
| 4999660 | 2020-05-15 04:09:24 |
| 4999661 | 2020-05-15 04:09:24 |
| 4999662 | 2020-05-15 04:09:24 |
| 4999663 | 2020-05-15 04:09:24 |
| 4999664 | 2020-05-15 04:09:24 |
| 4999665 | 2020-05-15 04:09:24 |
+---------+---------------------+
9 rows in set (4.10 sec)

加上limit 1后（速度提升一大半）：

mysql> select * from fa_ceshi where times="2020-05-15 04:09:24" limit 1;
+---------+---------------------+
| id      | times               |
+---------+---------------------+
| 4999657 | 2020-05-15 04:09:24 |
+---------+---------------------+
1 row in set (1.77 sec)

设置times为索引的情况下（有无limit差别不大）：

mysql> select * from fa_ceshi where times="2020-05-15 04:09:24";
+---------+---------------------+
| id      | times               |
+---------+---------------------+
| 4999657 | 2020-05-15 04:09:24 |
| 4999658 | 2020-05-15 04:09:24 |
| 4999659 | 2020-05-15 04:09:24 |
| 4999660 | 2020-05-15 04:09:24 |
| 4999661 | 2020-05-15 04:09:24 |
| 4999662 | 2020-05-15 04:09:24 |
| 4999663 | 2020-05-15 04:09:24 |
| 4999664 | 2020-05-15 04:09:24 |
| 4999665 | 2020-05-15 04:09:24 |
+---------+---------------------+
9 rows in set (0.00 sec)

在有索引的情况下就没必要使用limit，使用limit的目的就是为了防止全表扫描。

十一、减少重复索引和冗余索引

11.1 重复索引

重复索引是指在相同的列上按照相同的顺序创建的相同类型的索引。

例如：创建一个表，设置id为主键、唯一约束，又将id设置为单列索引。

《高性能mysql》：事实上，mysql的唯一限制和主键限制都是通过索引实现的，因此，上面这个例子就等于创建了三个重复的索引。通过没有理由这样做，除非是在同一列上创建不同的类型的索引来满足不同的需求。

11.2 冗余索引

如果创建了联合索引（A，B），在去创建索引A，那么索引A就属于冗余索引，因为A在联合索引（A，B）最左侧，能作为索引A来使用（这种冗余只是对B-tree索引来说的）。

但是如果在创建索引（B，A）或者索引B，则不是冗余索引，因为B不在索引（A，B）的最左侧。

另外，其他不同类型的索引（例如哈希索引或全文索引）也不会是B-tree索引的冗余索引，而无论覆盖索引的列是什么。

十二、where条件与order by一起使用

12.1 where条件字段与order by条件字段不一致的情况

其中，sname为单列索引，age为单列索引。

mysql> explain select * from student where sname="李关亮" order by age;
+----+-------------+---------+------+---------------+-------+---------+-------+------+----------------------------------------------------+
| id | select_type | table   | type | possible_keys | key   | key_len | ref   | rows | Extra                                              |
+----+-------------+---------+------+---------------+-------+---------+-------+------+----------------------------------------------------+
|  1 | SIMPLE      | student | ref  | sname         | sname | 63      | const |    1 | Using index condition; Using where; Using filesort |
+----+-------------+---------+------+---------------+-------+---------+-------+------+----------------------------------------------------+
1 row in set (0.00 sec)

发现出现了Using filesort。

关于FileSort排序：一般在内存中进行排序，占用CPU较多。如果待排结果较大，会产生临时文件I/O到磁盘进行排序，效率较低。

进行改进，将sname，age设置为联合索引：

mysql> explain select * from student where sname="李关亮" order by age;
+----+-------------+---------+------+-----------------+-----------------+---------+-------+------+------------------------------------+
| id | select_type | table   | type | possible_keys   | key             | key_len | ref   | rows | Extra                              |
+----+-------------+---------+------+-----------------+-----------------+---------+-------+------+------------------------------------+
|  1 | SIMPLE      | student | ref  | index_sname_age | index_sname_age | 63      | const |    1 | Using index condition; Using where |
+----+-------------+---------+------+-----------------+-----------------+---------+-------+------+------------------------------------+
1 row in set (0.00 sec)

发现FileSort排序已经消失，从而实现了索引覆盖。因此，使用联合索引在where条件字段与order by条件字段不一致的情况下能够提高效率，使用索引扫描。

12.2 where条件字段与order by条件字段一致的情况

其中，sname为单列索引，不使用where条件直接order by进行排序：

mysql> explain select * from student order by sname;
+----+-------------+---------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+---------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | student | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

直接全盘扫描，加上where条件后，实现了覆盖索引：

mysql> explain select * from student where sname="李关亮" order by sname;
+----+-------------+---------+------+---------------+-------+---------+-------+------+-----------------------+
| id | select_type | table   | type | possible_keys | key   | key_len | ref   | rows | Extra                 |
+----+-------------+---------+------+---------------+-------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | student | ref  | sname         | sname | 63      | const |    1 | Using index condition |
+----+-------------+---------+------+---------------+-------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

总结：加WHERE子句可避免全表扫描。使用ORDER BY子句提高排序速度。

---

十三、在多表关联查询时，两大原则

13.1 让小表在前，大表在后

第一张表会涉及到全表扫描，先扫小表，在扫描后面的大表，节省运算，以此提高效率。

13.2 让重复关联键少的表在前，重复关键键多的在后

关联左侧的表每有1条重复的关联键时底层就会多1次运算处理。具体解析参考：SQL Join连接大小表在前在后的重要性（小表在前提高执行效率）

十四、总结

如有错误，敬请更正。衷心感谢，不胜荣幸。

关于explain：mysql explain用法和结果的含义