1.真的需要索引嗎？
MySQL索引的建立對於MySQL的高效運行是很重要的，索引可以大大提高MySQL的檢索速度。
打個比方，如果合理的設計且使用索引的MySQL是一輛蘭博基尼的話，那麼沒有設計和使用索引的MySQL就是一個人力三輪車。
拿漢語字典的目錄頁（索引）打比方，我們可以按拼音、筆畫、偏旁部首等排序的目錄（索引）快速查找到需要的字。
索引分單列索引和組合索引。單列索引，即一個索引只包含單個列，一個表可以有多個單列索引，但這不是組合索引。組合索引，即一個索引包含多個列。
上面都在說使用索引的好處，但過多的使用索引將會造成濫用。因此索引也會有它的缺點：雖然索引大大提高了查詢速度，同時卻會降低更新表的速度，如對錶進行INSERT、UPDATE和DELETE。因爲更新表時，MySQL不僅要保存數據，還要保存一下索引文件。
建立索引會佔用磁盤空間的索引文件。
2.什麼是索引
索引（在 MySQL 中也叫“鍵key”）是存儲引擎快速找到記錄的一種數據結構
——《高性能MySQL》
我們需要知道索引其實是一種數據結構，其功能是幫助我們快速匹配查找到需要的數據行，是數據庫性能優化最常用的工具之一。其作用相當於超市裏的導購員、書本里的目錄。
3.測試表
本篇文章，我們將從索引基礎開始，介紹什麼是索引以及索引的幾種類型，然後學習如何創建索引以及索引設計的基本原則。
本篇文章中用於測試索引創建的user表的結構如下：
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`gender` int(1) NOT NULL,
`age` int(3) NOT NULL,
`status` int(1) NOT NULL,
`remark` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 1 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;
4.索引類型
查看索引詳情
SHOW INDEX FROM table_name;
例如：
mysql> SHOW INDEX FROM user;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.03 sec)
4.1.主鍵索引
它是一種特殊的唯一索引，不允許有空值。一般是在建表的時候同時創建主鍵索引。
mysql> SHOW INDEX FROM user;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.03 sec)
注意：一個表只能有一個主鍵
4.2.唯一索引
唯一索引列的值必須唯一，但允許有空值。如果是組合索引，則列值的組合必須唯一。
創建唯一索引:
ALTER TABLE table_name ADD UNIQUE (column);
示例：
mysql> alter table user add unique(name);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEX FROM user;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | name | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.03 sec)
創建唯一組合索引：
ALTER TABLE table_name ADD UNIQUE (column1,column2);
示例：
mysql> ALTER TABLE user ADD UNIQUE unique_name_age (name,age);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEX FROM user;
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | name | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | unique_name_age | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | unique_name_age | 2 | age | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.03 sec)
4.3.普通索引
最基本的索引，它沒有任何限制。
創建普通索引：
ALTER TABLE table_name ADD INDEX index_name (column);
示例：
mysql> alter table user add index index_name(name);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEX FROM user;
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | name | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | unique_name_age | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | unique_name_age | 2 | age | A | 0 | NULL | NULL | | BTREE | | |
| user | 1 | index_name | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
5 rows in set (0.03 sec)
4.4.組合索引
組合索引，即一個索引包含多個列。多用於避免回表查詢。
創建組合索引：
ALTER TABLE table_name ADD INDEX index_name(column1, column2, column3);
示例：
mysql> alter table user add index index_name_age(name,age);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEX FROM user;
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | name | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | unique_name_age | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
| user | 0 | unique_name_age | 2 | age | A | 0 | NULL | NULL | | BTREE | | |
| user | 1 | index_name | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
| user | 1 | index_name_age | 1 | name | A | 0 | NULL | NULL | | BTREE | | |
| user | 1 | index_name_age | 2 | age | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
7 rows in set (0.05 sec)
4.5.全文索引
全文索引（也稱全文檢索）是目前搜索引擎使用的一種關鍵技術。
創建全文索引
ALTER TABLE table_name ADD FULLTEXT (column);
示例：
mysql> ALTER TABLE user ADD FULLTEXT (remark);
Database changed
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW INDEX FROM user;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| user | 1 | remark | 1 | remark | NULL | NULL | NULL | NULL | YES | FULLTEXT | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set
備註: 如果是InnoDB，改爲MyISAM，InnoDB不支持FULLTEXT類型的索引
5.刪除索引
索引一經創建不能修改，如果要修改索引，只能刪除重建。
刪除索引：
DROP INDEX index_name ON table_name;
6.索引設計的原則
1.適合索引的列是出現在where子句中的列，或者連接子句中指定的列
2.基數較小的類，索引效果較差，沒有必要在此列建立索引
3.使用短索引，如果對長字符串列進行索引，應該指定一個前綴長度，這樣能夠節省大量索引空間
CREATE INDEX indexName ON mytable(username(length));
如果是CHAR，VARCHAR類型，length可以小於字段實際長度；如果是BLOB和TEXT類型，必須指定 length。
4.不要過度索引。索引需要額外的磁盤空間，並降低寫操作的性能。在修改表內容的時候，索引會進行更新甚至重構，索引列越多，這個時間就會越長。所以只保持需要的索引有利於查詢即可。