MySQL 提供 MyISAM 、 InnoDB 、 memory(heap) 等多種存儲引擎。每種存儲引擎對于索引的支持以及實現(xiàn)都不盡相同，
本文主要討論 InnoDB 引擎相關(guān)的索引應(yīng)用。

為何使用索引

索引用于快速找出在某個列中有一特定值的行。在查詢時如果沒有應(yīng)用索引，MySQL 將不得不掃描表以找出符合條件的數(shù)據(jù)項，我們知道，IO操作是非常耗時的。

建立索引的幾個原則

• 盡量使用唯一索引，對于有唯一值的列索引效果最好，對于像性別只有很少的值的列索引效果就不明顯。

• 索引長度盡量短，這樣做有幾個好處，首先短的索引可以節(jié)省索引空間，也會使查找的速度得到提升。對于較短的鍵值，索引高速緩存中的塊能容納更多的鍵值，MySQL也可以在內(nèi)存中容納更多的值。

• 太長的列，可以選擇只建立部分索引

• 更新非常頻繁的數(shù)據(jù)不適宜建索引

• 利用最左前綴原則，比如建立了一個聯(lián)合索引(a,b,c)，那么其實我們可利用的索引就有(a), (a,b), (a,b,c)

• 不要過多創(chuàng)建索引，首先過多的索引會占用更多的空間，而且每次增、刪、改操作都會重建索引，而且過多索引也會增加之后的優(yōu)化復(fù)雜度

• 盡量擴展索引，比如現(xiàn)有索引(a)，現(xiàn)在我又要對(a,b)進行索引，不需要再建一個索引(a,b)

• 請注意，一次查詢是不能應(yīng)用多個索引的

• <，<=，=，>，>=，BETWEEN，IN 可用到索引，<>，not in ，!= 則不行

• like "xxxx%" 是可以用到索引的，like "%xxxx" 則不行(like "%xxx%" 同理)

• NULL會使索引的效果大打折扣

淺談聯(lián)合索引

首先因為 InnoDB 的數(shù)據(jù)文件本身要按主鍵聚集，所以InnoDB要求表必須有主鍵（MyISAM可以沒有），如果沒有顯式指定，則MySQL系統(tǒng)會自動選擇一個可以唯一標(biāo)識數(shù)據(jù)記錄的列作為主鍵，如果不存在這種列，則MySQL自動為InnoDB表生成一個隱含字段作為主鍵。主鍵對于 InnoDB 的索引結(jié)構(gòu)是十分重要的。

InnoDB 引擎的索引是使用 B+樹 實現(xiàn)索引結(jié)構(gòu)的，當(dāng)我們建立一個聯(lián)合索引(a, b, c)時，B+樹將按照從左至右來建立搜索樹，然后檢索時B+樹將先比較 a 然后再其基礎(chǔ)上比較 b 和 c 。不難看出如果我們的搜索條件中只有 a 和 c ，將不能使用完整的(a, b, c)索引，如果我們的搜索條件中沒有 a 那么這條查詢將不會用上索引，這其實就是最左前綴特性。

** 接下來我們來看下聯(lián)合索引應(yīng)用時的幾種情況： **

desc users;

+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| rname | varchar(50) | NO   |     | NULL    |                |
| rdesc | varchar(50) | NO   |     | NULL    |                |
| age   | int(11)     | NO   | MUL | NULL    |                |
| card  | int(5)      | YES  |     | NULL    |                |
+-------+-------------+------+-----+---------+----------------+

show index from users;

+-------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name      | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| users |          0 | PRIMARY       |            1 | id          | A         |          15 |     NULL | NULL   |      | BTREE      |         |               |
| users |          1 | age_name_desc |            1 | age         | A         |          15 |     NULL | NULL   |      | BTREE      |         |               |
| users |          1 | age_name_desc |            2 | rname       | A         |          15 |       10 | NULL   |      | BTREE      |         |               |
| users |          1 | age_name_desc |            3 | rdesc       | A         |          15 |       15 | NULL   |      | BTREE      |         |               |
+-------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

• 全列匹配

explain select * from users where age = 11 and rname = "asd" and rdesc = "asd";

+----+-------------+-------+------+---------------+---------------+---------+-------------------+------+-------------+
| id | select_type | table | type | possible_keys | key           | key_len | ref               | rows | Extra       |
+----+-------------+-------+------+---------------+---------------+---------+-------------------+------+-------------+
|  1 | SIMPLE      | users | ref  | age_name_desc | age_name_desc | 83      | const,const,const |    1 | Using where |
+----+-------------+-------+------+---------------+---------------+---------+-------------------+------+-------------+

以上 explain 的結(jié)果可以看出，當(dāng)對索引中所有列進行精確匹配的時候，可以用到完整索引。

explain select * from users where  rname = "asd" and age = 11 and rdesc = "asd";

+----+-------------+-------+------+---------------+---------------+---------+-------------------+------+-------------+
| id | select_type | table | type | possible_keys | key           | key_len | ref               | rows | Extra       |
+----+-------------+-------+------+---------------+---------------+---------+-------------------+------+-------------+
|  1 | SIMPLE      | users | ref  | age_name_desc | age_name_desc | 83      | const,const,const |    1 | Using where |
+----+-------------+-------+------+---------------+---------------+---------+-------------------+------+-------------+

按照B+樹的結(jié)構(gòu)聯(lián)合索引本是對順序十分敏感的，但是從以上結(jié)果可看出調(diào)整順序并沒有影響到索引的選用，這是因為MySQL的查詢優(yōu)化器會自動調(diào)整where子句的條件順序以使用適合的索引。

• 部分匹配（左前綴）

explain select * from users where age = 11;

+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key           | key_len | ref   | rows | Extra |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------+
|  1 | SIMPLE      | users | ref  | age_name_desc | age_name_desc | 4       | const |    1 | NULL  |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------+

從上述結(jié)果可看出當(dāng)精確匹配最左前綴的列時，是可以用到索引的，但是 key_len = 4，只用到了第一列前綴索引。

explain select * from users where age = 11 and rdesc = "asd";

+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key           | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+
|  1 | SIMPLE      | users | ref  | age_name_desc | age_name_desc | 4       | const |    1 | Using where |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+

上述查詢?nèi)笔Я藃name列，可以看出當(dāng)缺失中間列時將不能使用完整的聯(lián)合索引。查詢只用到了最左部分索引，而rdesc由于沒有rname無法和左前綴銜接。

explain select * from users where rname = "asd";

+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | users | ALL  | NULL          | NULL | NULL    | NULL |   15 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

上述查詢由于查詢條件不是最左前綴，將不能使用聯(lián)合索引age_name_desc，建立聯(lián)合索引時順序是很重要的，必須在建索引前考慮清楚。

• 非精確查詢

explain select * from users where age = 11 and rname like 'sad%';

+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | Extra                              |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+
|  1 | SIMPLE      | users | range | age_name_desc | age_name_desc | 36      | NULL |    1 | Using index condition; Using where |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+

explain select * from users where age = 11 and rname like 'sad%' and rdesc = 'asd';

+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | Extra                              |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+
|  1 | SIMPLE      | users | range | age_name_desc | age_name_desc | 83      | NULL |    1 | Using index condition; Using where |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+

可以看出like查詢當(dāng)通配符'%'不出現(xiàn)在開頭時，是可以應(yīng)用到索引的。

explain select * from users where age = 11 and rname like '%sad%';

+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key           | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+
|  1 | SIMPLE      | users | ref  | age_name_desc | age_name_desc | 4       | const |    1 | Using where |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+

通配符'%'出現(xiàn)在開頭則不行

接下來我們看看范圍查詢

explain select * from users where age = 61 and rname < 'fasd';

+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | Extra                              |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+
|  1 | SIMPLE      | users | range | age_name_desc | age_name_desc | 36      | NULL |    1 | Using index condition; Using where |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+------------------------------------+

• 函數(shù)和表達式

explain select * from users where (age + 1) = 33;

+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | users | ALL  | NULL          | NULL | NULL    | NULL |   15 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

當(dāng)查詢條件中出現(xiàn)函數(shù)或表達式時，將不能應(yīng)用索引。寫查詢語句時要盡量避免表達式和函數(shù)的出現(xiàn)。

指導(dǎo)性建議

建議在選擇性高的列上建立索引，所謂索引的選擇性，是指不重復(fù)的索引值與表記錄數(shù)的比值：

Index Selectivity = Cardinality / Count

顯然選擇性的取值范圍為(0, 1]，選擇性越高的索引價值越大。
比例越大我們掃描的記錄數(shù)越少，唯一鍵的區(qū)分度是1，而一些狀態(tài)、性別字段可能在大數(shù)據(jù)面前區(qū)分度就趨近于0

select count(distinct(rname))/count(*) as selectivity from users;

+-------------+
| selectivity |
+-------------+
|      0.9333 |
+-------------+

有一種與索引選擇性有關(guān)的索引優(yōu)化策略叫做前綴索引，就是用列的前綴代替整個列作為索引key，當(dāng)前綴長度合適時，可以做到既使得前綴索引的選擇性接近全列索引，同時因為索引key變短而減少了索引文件的大小和維護開銷。

select count(distinct(left(rname, 3)))/count(*) as selectivity from users;

+-------------+
| selectivity |
+-------------+
|      0.7333 |
+-------------+

select count(distinct(left(rname, 5)))/count(*) as selectivity from users;

+-------------+
| selectivity |
+-------------+
|      0.9333 |
+-------------+

可以看到，當(dāng)把前綴取到5時selectivity值就和完整的selectivity值一樣了，這樣可以大幅度減小索引所占用的空間，而且相應(yīng)的查詢速度也會有一定提升。