MySQL中Explain初识

2022-11-19 19:14:49

Index

MySQL索引的基本操作

CREATE INDEX idx_price on OrderItems(item_price); 
ALTER TABLE OrderItems DROP INDEX idx_order_num_price;

 

Explain

MySQL 提供了一个 EXPLAIN 命令, 它可以对 SELECT 语句进行分析, 并输出 SELECT 执行的详细信息, 以供开发人员针对性优化。EXPLAIN 命令用法十分简单, 在 SELECT 语句前加上 Explain 就可以了。

Explain各列的含义如下:

id: 每个 SELECT 都会自动分配一个唯一的标识符
select_type: SELECT 查询的类型
table: 查询的是哪个表
partitions: 匹配的分区
type: 访问类型
possible_keys: 此次查询中可能选用的索引
key: 此次查询中确切使用到的索引
ref: 哪个字段或常数与 key 一起被使用
rows: 显示此查询一共扫描了多少行 这个是一个估计值
filtered: 表示此查询条件所过滤的数据的百分比
extra: 额外的信息

其中Type是重点关注的字段，用以快速评价查询语句性能，常见type性能从差到好如下：

All < Index < Range < Ref < const

 

Demo

以OrderItems表为例：

| OrderItems | CREATE TABLE `OrderItems` (
  `order_num` int(11) NOT NULL,
  `order_item` int(11) NOT NULL,
  `prod_id` char(10) NOT NULL,
  `quantity` int(11) NOT NULL,
  `item_price` decimal(8,2) NOT NULL,
  PRIMARY KEY (`order_num`,`order_item`),
  KEY `FK_OrderItems_Products` (`prod_id`),
  KEY `idx_price` (`item_price`),
  KEY `idx_order_num_price` (`order_num`,`item_price`),
  constRAINT `FK_OrderItems_Orders` FOREIGN KEY (`order_num`) REFERENCES `Orders` (`order_num`),
  CONSTRAINT `FK_OrderItems_Products` FOREIGN KEY (`prod_id`) REFERENCES `Products` (`prod_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |


mysql> select * from OrderItems;
+-----------+------------+---------+----------+------------+
| order_num | order_item | prod_id | quantity | item_price |
+-----------+------------+---------+----------+------------+
|     20005 |          1 | BR01    |      100 |       5.49 |
|     20005 |          2 | BR03    |      100 |      10.99 |
|     20006 |          1 | BR01    |       20 |       5.99 |
|     20006 |          2 | BR02    |       10 |       8.99 |
|     20006 |          3 | BR03    |       10 |      11.99 |
|     20007 |          1 | BR03    |       50 |      11.49 |
|     20007 |          2 | BNBG01  |      100 |       2.99 |
|     20007 |          3 | BNBG02  |      100 |       2.99 |
|     20007 |          4 | BNBG03  |      100 |       2.99 |
|     20007 |          5 | RGAN01  |       50 |       4.49 |
|     20008 |          1 | RGAN01  |        5 |       4.99 |
|     20008 |          2 | BR03    |        5 |      11.99 |
|     20008 |          3 | BNBG01  |       10 |       3.49 |
|     20008 |          4 | BNBG02  |       10 |       3.49 |
|     20008 |          5 | BNBG03  |       10 |       3.49 |
|     20009 |          1 | BNBG01  |      250 |       2.49 |
|     20009 |          2 | BNBG02  |      250 |       2.49 |
|     20009 |          3 | BNBG03  |      250 |       2.49 |
+-----------+------------+---------+----------+------------+
18 rows in set (0.03 sec)

举例说明几个Explain type的查询：

all，直接全表查询

mysql> explain select item_price from OrderItems;
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | OrderItems | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   18 |   100.00 | NULL  |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+

index，利用索引

mysql> explain select order_num from OrderItems;    
+----+-------------+------------+------------+-------+---------------+------------------------+---------+------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys | key                    | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+---------------+------------------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | OrderItems | NULL       | index | NULL          | FK_OrderItems_Products | 30      | NULL |   18 |   100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+------------------------+---------+------+------+----------+-------------+

range，对索引进行范围查询，多见于between/in/<>等关键字

mysql> explain select order_num from OrderItems where order_num between 20003 and 20005;
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table      | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | OrderItems | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where; Using index |
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+

ref，使用普通索引

mysql> explain select order_num from OrderItems where order_num = 20005;
+----+-------------+------------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table      | partitions | type | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | OrderItems | NULL       | ref  | PRIMARY       | PRIMARY | 4       | const |    2 |   100.00 | Using index |
+----+-------------+------------+------------+------+---------------+---------+---------+-------+------+----------+-------------+

consts 单表中最多只有一个匹配行（主键或者唯一索引），在优化阶段即可读取到数。

 

Optimization

通过分析Explain语句结果可以优化查询性能。一般关注点有：一是type往好的方向优化，二是有order by时，尽量不要在extra中出现Using filesort。

type优化，如 阿里巴巴Java编程规范 中定义的那样，一般不允许all和index的查询，会极大影响性能。尽量优化至range以上。方法一般就是建索引，不要为了节省插入性能而去缩减必要的索引。

mysql> CREATE INDEX idx_price on OrderItems(item_price);

然后再执行关于item_price的查询：

mysql> explain select order_num from OrderItems where item_price between 5 and 8;
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table      | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | OrderItems | NULL       | range | idx_price     | idx_price | 4       | NULL |    2 |   100.00 | Using where; Using index |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

看到type位置变为range，possible_keys和key的位置出现了我们新建的索引

去除Using filesort

执行下列查询

mysql> explain select * from OrderItems where order_num = 20003 order by item_price;         
+----+-------------+------------+------------+------+---------------+---------+---------+-------+------+----------+-----------------------------+
| id | select_type | table      | partitions | type | possible_keys | key     | key_len | ref   | rows | filtered | Extra                       |
+----+-------------+------------+------------+------+---------------+---------+---------+-------+------+----------+-----------------------------+
|  1 | SIMPLE      | OrderItems | NULL       | ref  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using where; Using filesort |
+----+-------------+------------+------------+------+---------------+---------+---------+-------+------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)

使用了fileSort。我们建立索引并再次执行查询如下：

mysql> CREATE INDEX idx_order_num_price on OrderItems(order_num, item_price);

mysql> explain select * from OrderItems where order_num = 20003 order by item_price;                
+----+-------------+------------+------------+------+-----------------------------+---------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table      | partitions | type | possible_keys               | key                 | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+------------+------------+------+-----------------------------+---------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | OrderItems | NULL       | ref  | PRIMARY,idx_order_num_price | idx_order_num_price | 4       | const |    1 |   100.00 | Using index condition |
+----+-------------+------------+------------+------+-----------------------------+---------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.01 sec)

如果建立了多重索引A-B(A和B是column name)，那么查询语句的where clause中仅使用了A也是可以利用该A-B索引的。事实上，只要查询条件从左至右依次匹配某索引，都是可以利用的。

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Problems

另外遇到两个关于索引有序性使用的小坑。

查询语句中的字段匹配索引的前半部分，但如果它们是用于in/between，索引失效

比如在建立了order_num/item_price索引的情况下，还是会fileSort：

mysql> explain select order_num from OrderItems where order_num between 20003 and 20005 order by item_price;
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------------+
| id | select_type | table      | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                       |
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------------+
|  1 | SIMPLE      | OrderItems | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where; Using filesort |
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------------+

单一Order时使用索引不分升降序，但如果对多字段排序，则要求索引顺序和查询语句’一致’

mysql> explain select item_price from OrderItems ORDER BY order_num, item_price;  
+----+-------------+------------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys | key                 | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | OrderItems | NULL       | index | NULL          | idx_order_num_price | 8       | NULL |   18 |   100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select item_price from OrderItems ORDER BY order_num, item_price desc;
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+
| id | select_type | table      | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                       |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+
|  1 | SIMPLE      | OrderItems | NULL       | index | NULL          | idx_price | 4       | NULL |   18 |   100.00 | Using index; Using filesort |
+----+-------------+------------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select item_price from OrderItems ORDER BY order_num desc, item_price desc;
+----+-------------+------------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys | key                 | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | OrderItems | NULL       | index | NULL          | idx_order_num_price | 8       | NULL |   18 |   100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

如果理解了MySQL索引的物理实现（B+ Tree)，这些应该就比较容易理解了（TODO）。

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