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。
