MySQL之查询优化方式（笔记）

1.COUNT()

　　对COUNT的优化可以通过下面的SQL实现

　　

mysql> select count(gnp<10000 or null) as  '<<<<',count(gnp>=10000 or null) as '>>>>' from country;
+------+------+
| <<<< | >>>> |
+------+------+
|  152 |   87 |
+------+------+
1 row in set (0.00 sec)

count(*)与count(cloumn)返回的值可能不一样，因为如果存在空的情况，count(*)也会计算在内

2.SUM()

对SUM（）的优化需要通过建立索引实现

　　

mysql> desc city;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   | MUL |         |                |
| CountryCode | char(3)  | NO   |     |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+
5 rows in set (0.03 sec)

mysql> create index popu_index on city(population);
Query OK, 4079 rows affected (0.05 sec)
Records: 4079  Duplicates: 0  Warnings: 0

mysql> explain select SUM(population) as  sum from city;
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key        | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+
|  1 | SIMPLE      | city  | index | NULL          | popu_index | 4       | NULL | 4079 | Using index |
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> select SUM(population) as  sum from city;
+------------+
| sum        |
+------------+
| 1429559884 |
+------------+
1 row in set (0.00 sec)

　3.子查询的优化

　　对于子查询的优化可以通过on实现

　

mysql> desc city;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   | MUL |         |                |
| CountryCode | char(3)  | NO   |     |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   | MUL | 0       |                |
+-------------+----------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

mysql> desc country;;
+----------------+---------------------------------------------------------------------------------------+------+-----+---------+--
| Field          | Type                                                                                  | Null | Key | Default | E
+----------------+---------------------------------------------------------------------------------------+------+-----+---------+--
| Code           | char(3)                                                                               | NO   | PRI |         |
| Name           | char(52)                                                                              | NO   |     |         |
| Continent      | enum('Asia','Europe','North America','Africa','Oceania','Antarctica','South America') | NO   |     | Asia    |
| Region         | char(26)                                                                              | NO   |     |         |
| SurfaceArea    | float(10,2)                                                                           | NO   |     | 0.00    |
| IndepYear      | smallint(6)                                                                           | YES  |     | NULL    |
| Population     | int(11)                                                                               | NO   |     | 0       |
| LifeExpectancy | float(3,1)                                                                            | YES  |     | NULL    |
| GNP            | float(10,2)                                                                           | YES  |     | NULL    |
| GNPOld         | float(10,2)                                                                           | YES  |     | NULL    |
| LocalName      | char(45)                                                                              | NO   |     |         |
| GovernmentForm | char(45)                                                                              | NO   |     |         |
| HeadOfState    | char(60)                                                                              | YES  |     | NULL    |
| Capital        | int(11)                                                                               | YES  |     | NULL    |
| Code2          | char(2)                                                                               | NO   |     |         |
+----------------+---------------------------------------------------------------------------------------+------+-----+---------+--
15 rows in set (0.01 sec)

ERROR:
No query specified

mysql> explain select t.* from city t where t.countrycode in( select code from country);
+----+-------------+---------+--------+---------------+---------+---------+---------------------+------+-------------+
| id | select_type | table   | type   | possible_keys | key     | key_len | ref                 | rows | Extra       |
+----+-------------+---------+--------+---------------+---------+---------+---------------------+------+-------------+
|  1 | SIMPLE      | t       | ALL    | NULL          | NULL    | NULL    | NULL                | 4079 | NULL        |
|  1 | SIMPLE      | country | eq_ref | PRIMARY       | PRIMARY | 3       | world.t.CountryCode |    1 | Using index |
+----+-------------+---------+--------+---------------+---------+---------+---------------------+------+-------------+
2 rows in set (0.04 sec)

mysql> explain select t.* from city t join country t1 on t.countrycode = t1.code;
+----+-------------+-------+--------+---------------+---------+---------+---------------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                 | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+---------------------+------+-------------+
|  1 | SIMPLE      | t     | ALL    | NULL          | NULL    | NULL    | NULL                | 4079 | NULL        |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 3       | world.t.CountryCode |    1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+---------------------+------+-------------+
2 rows in set (0.00 sec)

　　这样的优化可能返回重复的行，去重需要使用关键字distinct

4.Group  by优化

正常执行情况下如下

　　

mysql> explain select t.* from city t join country t1 on t.countrycode = t1.code group by t.id;
+----+-------------+-------+--------+---------------+---------+---------+---------------------+------+---------------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                 | rows | Extra                           |
+----+-------------+-------+--------+---------------+---------+---------+---------------------+------+---------------------------------+
|  1 | SIMPLE      | t     | ALL    | NULL          | NULL    | NULL    | NULL                | 4079 | Using temporary; Using filesort |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 3       | world.t.CountryCode |    1 | Using index                     |
+----+-------------+-------+--------+---------------+---------+---------+---------------------+------+---------------------------------+
2 rows in set (0.02 sec)

　　可以看到了使用临时表还有文件排序的方式来实现，那么我们应该怎么减少IO已经对资源的消耗呢，通过下面方式可以看出

mysql> explain select t.* from city t join (select code from country) t1 on t.countrycode = t1.code;
+----+-------------+------------+-------+---------------+-------------+---------+---------------------+------+-------------+
| id | select_type | table      | type  | possible_keys | key         | key_len | ref                 | rows | Extra       |
+----+-------------+------------+-------+---------------+-------------+---------+---------------------+------+-------------+
|  1 | PRIMARY     | t          | ALL   | NULL          | NULL        | NULL    | NULL                | 4079 | NULL        |
|  1 | PRIMARY     | <derived2> | ref   | <auto_key0>   | <auto_key0> | 3       | world.t.CountryCode |   10 | Using index |
|  2 | DERIVED     | country    | index | NULL          | PRIMARY     | 3       | NULL                |  239 | Using index |
+----+-------------+------------+-------+---------------+-------------+---------+---------------------+------+-------------+
3 rows in set (0.00 sec)

　　这样就可以避免了文件排序还有使用该临时表的情况。

5.limit操作方式的优化

　　使用之前：

mysql> explain select * from city order by name limit  100,10;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | city  | ALL  | NULL          | NULL | NULL    | NULL | 4079 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

　　可以看到使用了文件排序，这样会造成性能上的低下。

　　优化方式：可以通过主键或者是含有索引的列进行order by  操作；

　　

mysql> create index name_index  on city(name);

mysql> explain select name from city order by name limit  100,10;
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key        | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+
|  1 | SIMPLE      | city  | index | NULL          | name_index | 35      | NULL |  110 | Using index |
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+
1 row in set (0.00 sec)

　　可以发现只扫描了35行而且使用了索引但是效率增加了很多。