index details
what is an index
An index is a way to speed up the retrieval of data in a table, just so that you don't want to scan the entire table.
The advantages of indexing
1. Greatly speed up finding data
2. Establish a unique index to ensure the uniqueness of each row of data in the data table
3. When performing grouping and sorting retrieval, the time can be significantly reduced
Disadvantages of Indexing
1. Reduce the speed of addition, deletion and modification
2. Creating and maintaining indexes requires our maintenance time
3. The index still needs to occupy a certain amount of physical space
Classification of indexes
By index method
- B-Tree Index
- Hash index
By index type
1. Ordinary index
Ordinary creation:
CREATE INDEX indexName ON mytable(username(length));
Modify table creation:
ALTER mytable ADD INDEX [indexName] ON (username(length))
Created when the table is created
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, INDEX [indexName] (username(length)) );
2. Unique Index
Guarantees uniqueness, but allows nulls
CREATE UNIQUE INDEX indexName ON mytable(username(length))
3. Primary key index
Guaranteed uniqueness, no null values are allowed, as the primary key, sometimes we will add auto-increment
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, PRIMARY KEY(ID) );
4. Full-text indexing
For full-text search, select Full Text as the index type
5. Composite Index
In order to further squeeze the efficiency of MySQL, it is necessary to consider building a composite index
ALTER TABLE mytable ADD INDEX name_city_age (name(10),city,age);
Establishing such a composite index is actually equivalent to establishing the following three sets of composite indexes:
usernname,city,age usernname,city usernname 为什么没有 city，age这样的组合索引呢？这是因为MySQL组合索引“最左前缀”的结果。简单的理解就是只从最左面的开始组合。并不是只要包含这三列的查询都会用到该组合索引，
索引的另一种分类
1. 直接创建索引和间接创建索引
直接创建索引： CREATE INDEX mycolumn_index ON mytable (myclumn)
间接创建索引：定义主键约束或者唯一性键约束，可以间接创建索引
2. 普通索引和唯一索引
普通索引：CREATE INDEX mycolumn_index ON mytable (myclumn)
唯一性索引：保证在索引列中的全部数据是唯一的，对聚簇索引和非聚簇索引都可以使用
CREATE UNIQUE COUSTERED INDEX myclumn_cindex ON mytable(mycolumn)
3. 单个索引和组合索引
单个索引：即非复合索引
复合索引：又叫组合索引，在索引建立语句中同时包含多个字段名，最多16个字段
CREATE INDEX name_index ON username(firstname,lastname)
4. 聚簇索引和非聚簇索引(聚集索引，群集索引)
聚簇索引：物理索引，与基表的物理顺序相同，数据值的顺序总是按照顺序排列
CREATE CLUSTERED INDEX mycolumn_cindex ON mytable(mycolumn) WITH ALLOW_DUP_ROW(允许有重复记录的聚簇索引)
非聚簇索引：CREATE UNCLUSTERED INDEX mycolumn_cindex ON mytable(mycolumn)
什么时候用索引
1. where 或 join 用到的字段加索引
注意MySQL只对<，<=，=，>，>=，BETWEEN，IN，以及某些时候的LIKE才会使用索引（like的时候关键字前面不能用%）
索引有效：
SELECT * FROM mytable WHERE username like'admin%'
索引无效：
SELECT * FROM mytable WHEREt Name like'%admin'
2. 不建议加索引的情况
1）重复且分布平均的字段，像sex这样的字段，只有男和女，就不需要加索引
2）数据太少，如果只有几行数据，就不要索引了
3）经常插入修改数据的，也不建议加索引
3. 当字段数据更新频率较低，查询使用频率较高并且存在大量重复值是建议使用聚簇索引
4. 经常同时存取多列，且每列都含有重复值可考虑建立组合索引
参考：
http://www.jb51.net/article/49346.htm
http://blog.sina.com.cn/s/blog_6a6eb42d0100kmyz.html
http://blog.csdn.net/xyh94233/article/details/6935669