Question

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)

以下是一些信息：

select count(*) from mytable

1 526 588行

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

解释：

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。

然而，查询无法更改，因为它来自另一个我没有动手的系统！

Answer 1

嗯，这个查询似乎很难设置索引：

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);

我的建议是从type开始，并在索引中包含其他列：mytable(type, url, data, pk)。这是一个覆盖索引，可能会稍微提高性能。

Answer 2

这不是答案。只有一些样本用于使用索引

<强>样品

小表

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)

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)

EXPLAIN 3查询（无索引）

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 | 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)

添加2个索引

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

使用索引进行分析

看看WHERE有2个字段，MySQL只使用一个INDEX

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COMPOSITE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

EXPLAIN查询

现在他们使用COMOSITE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OPTIMIZER合并指数

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

删除INDEX 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QUERY

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]>

Answer 3

MySQL只能在查询中使用一个INDEX。

所以有必要有复合索引。索引中字段顺序的最佳方式是主要减少结果集的字段，依此类推。

假设你有一张名字和生日的桌子，你有许多重复的名字，只有几个生日。所以然后创建索引像（生日，名字）mysql搜索首先记录搜索日，然后只在这个小结果名称

Answer 4

该查询的最佳索引是

INDEX(url, type) -- in that order

索引应该以＆＃34; =常量＆＃34;的任何列开头。（在这种情况下为url），然后再转到另一个更复杂的列（type IN ...）。 See my cookbook

使用索引优化MySQL查询（什么索引？）

4 个答案: