使用索引优化MySQL查询（什么索引？）

我正在努力优化以下查询，平均需要2000毫秒

select  count(pk)
    from  mytable
    where  (pk<>5
      and  url='test.png'
      and  (data=124578 or  (data is null and  pk=1234578)))
      and  type in (123,456,789,015,789) 

1 526 588条线路

show indexes in mytable

Table   non_unique  key_name            seq_in_index    column_name collation   cardinality
mytable 0           PRIMARY             1               PK          A           1405079     
mytable 1           data                1               data        A           1405079     
mytable 1           Media_Code_30       1               code        A           1405079     
mytable 1           codeVersionIDX_30   1               code        A           1405079     

说明：

id  select_type table   type        possible_keys   key     key_len ref     rows    extra
1   SIMPLE      mytable ref_or_null PRIMARY,data    data    9       const   635908  Using where

我真的不知道这是否足够优化，或者通过创建新索引（或复合索引）可以更好

---

但是，无法更改该查询，因为它来自另一个系统，而我不在该系统上

Hmmm，此查询似乎很难为以下内容设置索引：

select count(pk)
from mytable
where (pk <> 5 and url = 'test.png' and
       (data = 124578 or (data is null and pk = 1234578))
      ) and
      type in (123, 456, 789, 015, 789);

选择计数（主键）
从mytable
其中（pk 5和url='test.png'和
（数据=124578或（数据为空且pk=1234578））
)及
输入（123、456、789、015、789）；

---

我的建议是从

type

开始，并在索引中包括其他列：

mytable（type、url、data、pk）

。这是一个覆盖索引，可能会稍微提高性能。

Hmmm，此查询似乎很难为以下内容设置索引：

select count(pk)
from mytable
where (pk <> 5 and url = 'test.png' and
       (data = 124578 or (data is null and pk = 1234578))
      ) and
      type in (123, 456, 789, 015, 789);

选择计数（主键）
从mytable
其中（pk 5和url='test.png'和
（数据=124578或（数据为空且pk=1234578））
)及
输入（123、456、789、015、789）；

---

我的建议是从

type

开始，并在索引中包括其他列：

mytable（type、url、data、pk）

。这是一个覆盖索引，可能会稍微提高性能。

MySQL在一个查询中只能使用一个索引

因此，有必要建立综合指数。索引中字段顺序的最佳方法是字段，它可以最大程度地减少结果集，等等

---

假设你有一张有名字和生日的桌子，你有许多重复的名字，只有很少的生日。然后创建索引，比如（生日，名字）mysql搜索，首先搜索当天的记录，然后在这个小结果中，名字只能在一个查询中使用一个索引

因此，有必要建立综合指数。索引中字段顺序的最佳方法是字段，它可以最大程度地减少结果集，等等

---

假设你有一张有名字和生日的桌子，你有许多重复的名字，只有很少的生日。然后创建索引，比如（生日、名称）mysql搜索，首先搜索当天的记录，然后只在这个小结果中搜索名称，这不是答案。只有一些用于使用索引的示例

样本

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
| id   | select_type | table    | type        | possible_keys | key   | key_len | ref  | rows | Extra                                            |
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
|    1 | SIMPLE      | useindex | index_merge | n1,n2         | n2,n1 | 5,5     | NULL |    1 | Using intersect(n2,n1); Using where; Using index |
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [bb]>

小桌子

MariaDB [bb]> SHOW CREATE TABLE useindex\G
*************************** 1. row ***************************
       Table: useindex
Create Table: CREATE TABLE `useindex` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `num1` int(11) DEFAULT NULL,
  `num2` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2031586 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bb]> ALTER TABLE useindex
    -> ADD KEY n1 (num1),
    -> ADD KEY n2 (num2);
Query OK, 0 rows affected (12.08 sec)
Records: 0  Duplicates: 0  Warnings: 0

行数和样本数据

MariaDB [bb]> SELECT count(*) FROM USEindex;
+----------+
| count(*) |
+----------+
|  2000000 |
+----------+
1 row in set (0.43 sec)

MariaDB [bb]> SELECT * FROM useindex LIMIT 10;
+----+------+------+
| id | num1 | num2 |
+----+------+------+
|  1 |  405 |  906 |
|  2 |  656 |  656 |
|  3 |  906 |  407 |
|  4 |  156 |  157 |
|  5 |  406 |  908 |
|  6 |  656 |  659 |
|  7 |  907 |  409 |
|  8 |  157 |  160 |
|  9 |  407 |  910 |
| 10 |  657 |  661 |
+----+------+------+
10 rows in set (0.00 sec)

解释3个查询（无索引）

添加2个索引

MariaDB [bb]> SHOW CREATE TABLE useindex\G
*************************** 1. row ***************************
       Table: useindex
Create Table: CREATE TABLE `useindex` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `num1` int(11) DEFAULT NULL,
  `num2` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2031586 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bb]> ALTER TABLE useindex
    -> ADD KEY n1 (num1),
    -> ADD KEY n2 (num2);
Query OK, 0 rows affected (12.08 sec)
Records: 0  Duplicates: 0  Warnings: 0

用索引解释

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500;
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | useindex | ALL  | NULL          | NULL | NULL    | NULL | 1996444 | Using where |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num2 = 600;
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | useindex | ALL  | NULL          | NULL | NULL    | NULL | 1996444 | Using where |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | useindex | ALL  | NULL          | NULL | NULL    | NULL | 1996444 | Using where |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

查看带有2个字段的WHERE，MySQL是否只使用一个索引

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
|    1 | SIMPLE      | useindex | ref  | n1            | n1   | 5       | const | 2003 |       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num2 = 600;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
|    1 | SIMPLE      | useindex | ref  | n2            | n2   | 5       | const | 1993 |       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
|    1 | SIMPLE      | useindex | ref  | n1,n2         | n2   | 5       | const | 1993 | Using where |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

在两个字段上添加综合索引

MariaDB [bb]> ALTER TABLE useindex
    -> ADD KEY n12 (num1,num2);
Query OK, 0 rows affected (7.83 sec)
Records: 0  Duplicates: 0  Warnings: 0

再次解释查询

现在，他们使用共站点索引

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
|    1 | SIMPLE      | useindex | ref  | n1,n12        | n12  | 5       | const | 2003 | Using index |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num2 = 600;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
|    1 | SIMPLE      | useindex | ref  | n2            | n2   | 5       | const | 1993 |       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref         | rows | Extra       |
+------+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
|    1 | SIMPLE      | useindex | ref  | n1,n2,n12     | n12  | 10      | const,const |    1 | Using index |
+------+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

告诉优化器合并索引

MariaDB [bb]> SET optimizer_switch='index_merge=on';
Query OK, 0 rows affected (0.00 sec)

下跌指数n12

MariaDB [bb]> alter table useindex drop key n12;
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

使用MERGE运行一个查询

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
| id   | select_type | table    | type        | possible_keys | key   | key_len | ref  | rows | Extra                                            |
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
|    1 | SIMPLE      | useindex | index_merge | n1,n2         | n2,n1 | 5,5     | NULL |    1 | Using intersect(n2,n1); Using where; Using index |
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [bb]>

---

这不是答案。只有一些用于使用索引的示例

样本

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
| id   | select_type | table    | type        | possible_keys | key   | key_len | ref  | rows | Extra                                            |
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
|    1 | SIMPLE      | useindex | index_merge | n1,n2         | n2,n1 | 5,5     | NULL |    1 | Using intersect(n2,n1); Using where; Using index |
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [bb]>

小桌子

MariaDB [bb]> SHOW CREATE TABLE useindex\G
*************************** 1. row ***************************
       Table: useindex
Create Table: CREATE TABLE `useindex` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `num1` int(11) DEFAULT NULL,
  `num2` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2031586 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bb]> ALTER TABLE useindex
    -> ADD KEY n1 (num1),
    -> ADD KEY n2 (num2);
Query OK, 0 rows affected (12.08 sec)
Records: 0  Duplicates: 0  Warnings: 0

行数和样本数据

MariaDB [bb]> SELECT count(*) FROM USEindex;
+----------+
| count(*) |
+----------+
|  2000000 |
+----------+
1 row in set (0.43 sec)

MariaDB [bb]> SELECT * FROM useindex LIMIT 10;
+----+------+------+
| id | num1 | num2 |
+----+------+------+
|  1 |  405 |  906 |
|  2 |  656 |  656 |
|  3 |  906 |  407 |
|  4 |  156 |  157 |
|  5 |  406 |  908 |
|  6 |  656 |  659 |
|  7 |  907 |  409 |
|  8 |  157 |  160 |
|  9 |  407 |  910 |
| 10 |  657 |  661 |
+----+------+------+
10 rows in set (0.00 sec)

解释3个查询（无索引）

添加2个索引

MariaDB [bb]> SHOW CREATE TABLE useindex\G
*************************** 1. row ***************************
       Table: useindex
Create Table: CREATE TABLE `useindex` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `num1` int(11) DEFAULT NULL,
  `num2` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2031586 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

MariaDB [bb]> ALTER TABLE useindex
    -> ADD KEY n1 (num1),
    -> ADD KEY n2 (num2);
Query OK, 0 rows affected (12.08 sec)
Records: 0  Duplicates: 0  Warnings: 0

用索引解释

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500;
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | useindex | ALL  | NULL          | NULL | NULL    | NULL | 1996444 | Using where |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num2 = 600;
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | useindex | ALL  | NULL          | NULL | NULL    | NULL | 1996444 | Using where |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | useindex | ALL  | NULL          | NULL | NULL    | NULL | 1996444 | Using where |
+------+-------------+----------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

查看带有2个字段的WHERE，MySQL是否只使用一个索引

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
|    1 | SIMPLE      | useindex | ref  | n1            | n1   | 5       | const | 2003 |       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num2 = 600;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
|    1 | SIMPLE      | useindex | ref  | n2            | n2   | 5       | const | 1993 |       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
|    1 | SIMPLE      | useindex | ref  | n1,n2         | n2   | 5       | const | 1993 | Using where |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

在两个字段上添加综合索引

MariaDB [bb]> ALTER TABLE useindex
    -> ADD KEY n12 (num1,num2);
Query OK, 0 rows affected (7.83 sec)
Records: 0  Duplicates: 0  Warnings: 0

再次解释查询

现在，他们使用共站点索引

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
|    1 | SIMPLE      | useindex | ref  | n1,n12        | n12  | 5       | const | 2003 | Using index |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num2 = 600;
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref   | rows | Extra |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
|    1 | SIMPLE      | useindex | ref  | n2            | n2   | 5       | const | 1993 |       |
+------+-------------+----------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
| id   | select_type | table    | type | possible_keys | key  | key_len | ref         | rows | Extra       |
+------+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
|    1 | SIMPLE      | useindex | ref  | n1,n2,n12     | n12  | 10      | const,const |    1 | Using index |
+------+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

告诉优化器合并索引

MariaDB [bb]> SET optimizer_switch='index_merge=on';
Query OK, 0 rows affected (0.00 sec)

下跌指数n12

MariaDB [bb]> alter table useindex drop key n12;
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

使用MERGE运行一个查询

MariaDB [bb]> EXPLAIN SELECT * FROM useindex WHERE num1 = 500 AND num2 = 600;
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
| id   | select_type | table    | type        | possible_keys | key   | key_len | ref  | rows | Extra                                            |
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
|    1 | SIMPLE      | useindex | index_merge | n1,n2         | n2,n1 | 5,5     | NULL |    1 | Using intersect(n2,n1); Using where; Using index |
+------+-------------+----------+-------------+---------------+-------+---------+------+------+--------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [bb]>

---

该查询的最佳索引是

INDEX(url, type) -- in that order

---

索引应该以“=constant”的任何列开始（在本例中为“

url

”），然后再移动到一个更复杂的列（

键入…

）

该查询的最佳索引是

INDEX(url, type) -- in that order

---

索引应该以“=constant”的任何列开始（在本例中为“

url

”），然后再移动到一个更复杂的列（

键入…

）

什么时候MySQL可以在一个查询中只使用一个索引？有文件支持吗？我愿意相信你。20年来，我一直在谈论子查询。问一个问题，看看埃普兰说了什么。优化器查找可用索引并使用最佳索引或无索引，或者更新meSo没有说明“MySQL每个查询仅使用一个索引”的文档，或者我误解了您的意思，您的意思是“在OP发布的查询中，MySQL只能使用一个索引”？抱歉，我可以快速找到包含此信息的文档，但我也没有发现mysql可以使用更多。但我会在稍后搜索并发布一个链接。实际上，使用两个索引是非常罕见的，不值得考虑。MySQL对单个

SELECT

使用两个索引的唯一方法是从两个索引中的每一个获取ID，然后在某个临时空间中对列表进行“anding”（“intersect”）或“oring”（“union”）操作。所有这些都是庞大而缓慢的，因此很少能比简单地使用一个索引，然后在

WHERE

或（在

连接

的情况下，使用嵌套循环连接）上进行过滤来提高性能？有文件支持吗？我愿意相信你。20年来，我一直在谈论子查询。问一个问题，看看埃普兰说了什么。优化器查找可用索引并使用最佳索引或无索引，或者更新meSo没有说明“MySQL每个查询仅使用一个索引”的文档，或者我误解了您的意思，您的意思是“在OP发布的查询中，MySQL只能使用一个索引”？抱歉，我可以快速找到包含此信息的文档，但我也没有发现mysql可以使用更多。但我会在稍后搜索并发布一个链接。实际上，使用两个索引是非常罕见的，不值得考虑。MySQL对单个

SELECT

使用两个索引的唯一方法是从两个索引中的每一个获取ID，t