为什么索引使这个查询更慢？

Question

表有1 500 000条记录,其中1 250 000条记录为field ='z'.
我需要选择随机而不是'z'字段.

$random = mt_rand(1, 250000);  
$query = "SELECT field FROM table WHERE field != 'z' LIMIT $random, 1";

它工作正常.

然后我决定优化它并field在表格中编入索引.

结果很奇怪 - 它慢了~3次.我测试了它.
它为什么慢？是不是这样的索引应该让它更快？

我的ISAM

explain with index:  
id  select_type  table  type  possible_keys  key   key_len  ref  rows     Extra  
1   SIMPLE       table  range field          field 758      NULL 1139287  Using  

explain without index:  
id  select_type  table  type  possible_keys  key  key_len  ref  rows     Extra  
1   SIMPLE       table  ALL   NULL           NULL NULL     NULL 1484672  Using where

Answer 1

摘要

field由于b树的性质,问题在于索引不是一个好的候选者.

说明

假设你有一张表有500,000个硬币投掷的结果,其中投掷是1(头)或0(尾):

CREATE TABLE toss (
    id int NOT NULL AUTO_INCREMENT,
    result int NOT NULL DEFAULT '0',
    PRIMARY KEY ( id )
)

select result, count(*) from toss group by result order by result;
+--------+----------+
| result | count(*) |
+--------+----------+
|      0 |   250290 |
|      1 |   249710 |
+--------+----------+
2 rows in set (0.40 sec)

如果你想选择一个折腾(随机)折腾尾巴,那么你需要搜索你的桌子,挑选一个随机的起始位置.

select * from toss where result != 1 limit 123456, 1;
+--------+--------+
| id     | result |
+--------+--------+
| 246700 |      0 |
+--------+--------+
1 row in set (0.06 sec)

explain select * from toss where result != 1 limit 123456, 1;
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | toss  | ALL  | NULL          | NULL | NULL    | NULL | 500000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+

您看到您基本上按顺序搜索所有行以查找匹配项.

如果在toss字段上创建索引,则索引将包含两个值,每个值大约有250,000个条目.

create index foo on toss ( result );
Query OK, 500000 rows affected (2.48 sec)
Records: 500000  Duplicates: 0  Warnings: 0

select * from toss where result != 1 limit 123456, 1;
+--------+--------+
| id     | result |
+--------+--------+
| 246700 |      0 |
+--------+--------+
1 row in set (0.25 sec)

explain select * from toss where result != 1 limit 123456, 1;
+----+-------------+-------+-------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | toss  | range | foo           | foo  | 4       | NULL | 154565 | Using where |
+----+-------------+-------+-------+---------------+------+---------+------+--------+-------------+

现在您搜索的记录较少,但搜索时间从0.06增加到0.25秒.为什么？因为顺序扫描索引实际上比顺序扫描表的效率低,对于给定键具有大量行的索引.

我们来看看这个表上的索引:

show index from toss;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| toss  |          0 | PRIMARY  |            1 | id          | A         |      500000 |     NULL | NULL   |      | BTREE      |         |
| toss  |          1 | foo      |            1 | result      | A         |           2 |     NULL | NULL   |      | BTREE      |         |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

PRIMARY索引是一个很好的索引:有500,000行,有500,000个值.安排在BTREE中,您可以根据ID快速识别单行.

foo索引是一个错误的索引:有500,000行,但只有2个可能的值.对于BTREE来说,这几乎是最糟糕的情况 - 搜索索引的所有开销,仍然需要搜索结果.