MySQL前綴索引和索引選擇性

有時候需要索引很長的字符列，這會讓索引變得大且慢。通常可以索引開始的部分字符，這樣可以大大節約索引空間，從而提高索引效率。但這樣也會降低索引的選擇性。索引的選擇性是指不重復的索引值（也稱為基數，cardinality)和數據表的記錄總數的比值，范圍從1/#T到1之間。索引的選擇性越高則查詢效率越高，因為選擇性高的索引可以讓MySQL在查找時過濾掉更多的行。唯一索引的選擇性是1，這是最好的索引選擇性，性能也是最好的。

一般情況下某個前綴的選擇性也是足夠高的，足以滿足查詢性能。對於BLOB，TEXT，或者很長的VARCHAR類型的列，必須使用前綴索引，因為MySQL不允許索引這些列的完整長度。

訣竅在於要選擇足夠長的前綴以保證較高的選擇性，同時又不能太長（以便節約空間）。前綴應該足夠長，以使得前綴索引的選擇性接近於索引的整個列。換句話說，前綴的”基數“應該接近於完整的列的”基數“。

為了決定前綴的合適長度，需要找到最常見的值的列表，然后和最常見的前綴列表進行比較。下面的示例是mysql官方提供的示例數據庫

下載地址如下：

http://downloads.mysql.com/docs/sakila-db.zip

在示例數據庫sakila中並沒有合適的例子，所以從表city中生成一個示例表，這樣就有足夠數據進行演示：

mysql> select database();                                                           
+------------+
| database() |
+------------+
| sakila     |
+------------+
1 row in set (0.00 sec)

mysql> create table city_demo (city varchar(50) not null);                          
Query OK, 0 rows affected (0.02 sec)

mysql> insert into city_demo (city) select city from city;                          
Query OK, 600 rows affected (0.08 sec)
Records: 600  Duplicates: 0  Warnings: 0

mysql> insert into city_demo (city) select city from city_demo;
Query OK, 600 rows affected (0.07 sec)
Records: 600  Duplicates: 0  Warnings: 0

mysql> update city_demo set city = ( select city from city order by rand() limit 1);
Query OK, 1199 rows affected (0.95 sec)
Rows matched: 1200  Changed: 1199  Warnings: 0

mysql> 

因為這里使用了rand（）函數，所以你的數據會與我的不同，當然那不影響聰明的你。

首先找到最常見的城市列表：

mysql> select count(*) as cnt, city from city_demo group by city order by cnt desc limit 10;               
+-----+--------------+
| cnt | city         |
+-----+--------------+
|   8 | Garden Grove |
|   7 | Escobar      |
|   7 | Emeishan     |
|   6 | Amroha       |
|   6 | Tegal        |
|   6 | Lancaster    |
|   6 | Jelets       |
|   6 | Ambattur     |
|   6 | Yingkou      |
|   6 | Monclova     |
+-----+--------------+
rows in set (0.01 sec)

mysql>

注意到查詢結果，上面每個值都出現了6-8次。現在查找到頻繁出現的城市前綴。先從3個前綴字母開始，然后4個，5個，6個：

mysql> select count(*) as cnt,left(city,3) as pref from city_demo group by pref order by cnt desc limit 10;
+-----+------+
| cnt | pref |
+-----+------+
|  25 | San  |
|  15 | Cha  |
|  12 | Bat  |
|  12 | Tan  |
|  11 | al-  |
|  11 | Gar  |
|  11 | Yin  |
|  10 | Kan  |
|  10 | Sou  |
|  10 | Bra  |
+-----+------+
10 rows in set (0.00 sec)

mysql> select count(*) as cnt,left(city,4) as pref from city_demo group by pref order by cnt desc limit 10; 
+-----+------+
| cnt | pref |
+-----+------+
|  12 | San  |
|  10 | Sout |
|   8 | Chan |
|   8 | Sant |
|   8 | Gard |
|   7 | Emei |
|   7 | Esco |
|   6 | Ying |
|   6 | Amro |
|   6 | Lanc |
+-----+------+
10 rows in set (0.01 sec)

mysql> select count(*) as cnt,left(city,5) as pref from city_demo group by pref order by cnt desc limit 10; 
+-----+-------+
| cnt | pref  |
+-----+-------+
|  10 | South |
|   8 | Garde |
|   7 | Emeis |
|   7 | Escob |
|   6 | Amroh |
|   6 | Yingk |
|   6 | Moncl |
|   6 | Lanca |
|   6 | Jelet |
|   6 | Tegal |
+-----+-------+
10 rows in set (0.01 sec)

 

mysql> select count(*) as cnt,left(city,6) as pref from city_demo group by pref order by cnt desc limit 10; 
+-----+--------+
| cnt | pref   |
+-----+--------+
|   8 | Garden |
|   7 | Emeish |
|   7 | Escoba |
|   6 | Amroha |
|   6 | Yingko |
|   6 | Lancas |
|   6 | Jelets |
|   6 | Tegal  |
|   6 | Monclo |
|   6 | Ambatt |
+-----+--------+
rows in set (0.00 sec)

mysql>

通過上面改變不同前綴長度發現，當前綴長度為6時，這個前綴的選擇性就接近完整列的選擇性了。甚至是一樣的。

 

 

 

 

當然還有另外更方便的方法，那就是計算完整列的選擇性，並使其前綴的選擇性接近於完整列的選擇性。下面顯示如何計算完整列的選擇性：

mysql> select count(distinct city) / count(*) from city_demo;
+---------------------------------+
| count(distinct city) / count(*) |
+---------------------------------+
|                          0.4283 |
+---------------------------------+
row in set (0.05 sec)

mysql>

可以在一個查詢中針對不同前綴長度的選擇性進行計算，這對於大表非常有用，下面給出如何在同一個查詢中計算不同前綴長度的選擇性：

mysql> select count(distinct left(city,3))/count(*) as sel3, -> count(distinct left(city,4))/count(*) as sel4, -> count(distinct left(city,5))/count(*) as sel5, -> count(distinct left(city,6))/count(*) as sel6 -> from city_demo; +--------+--------+--------+--------+
| sel3   | sel4   | sel5   | sel6   |
+--------+--------+--------+--------+
| 0.3367 | 0.4075 | 0.4208 | 0.4267 |
+--------+--------+--------+--------+
1 row in set (0.01 sec) mysql> 

可以看見當索引前綴為6時的基數是0.4267，已經接近完整列選擇性0.4283。

在上面的示例中，已經找到了合適的前綴長度，下面創建前綴索引：

mysql> alter table city_demo add key (city(6));
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0

 

mysql> explain select * from city_demo where city like 'Jinch%';
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | city_demo | range | city          | city | 20      | NULL |    2 | Using where |
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

可以看見正確使用剛創建的索引。

前綴索引是一種能使索引更小，更快的有效辦法，但另一方面也有其缺點：

mysql無法使用其前綴索引做ORDER BY和GROUP BY，也無法使用前綴索引做覆蓋掃描。

 

 

轉自:  http://www.cnblogs.com/gomysql