7.15 Day21---MySQL----Index

目录

索引：

索引的优缺点:

Syntax for creating and dropping indexes：

创建

删除

索引的数据结构：

Evolution of data structures：

二叉树：

平衡二叉树-AVL树：

多路平衡二叉树-B树：

 NYSQLHow the index is designed：

说说索引：

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索引：

索引的优缺点:

时间上

Indexes can greatly improve the speed of queries

At the expense of the speed of additions, deletions and changes,Because in order to ensure the orderliness of the index when adding, deleting and modifying,需要动态维护索引

空间方面：索引需要占物理空间.

索引的分类

主键索引：关键字primary key

唯一索引：关键字 unique

普通索引(非唯一)：关键字 index

全文索引：关键字 fulltext

聚簇索引=主键索引=一级索引

非聚簇索引=（唯一索引,普通索引）=二级索引

Frm文件：表结构文件

Ibd文件：表数据文件

Create drop alter-操作frm文件

Insert update delete -操作ibd文件

Only the primary key index is called a clustered index,A non-primary key index is called a non-clustered index

Innodb不支持全文索引

创建删除索引的语法

Clustered indexes are native to the table,We create non-clustered indexes ourselves,如果表没有主键.Then he will select the hidden column by defaultrowid 来创建聚簇索引

Syntax for creating and dropping indexes：

(聚簇索引=The primary key index comes with the table,We create non-clustered indexes ourselves)

（如果表没有主键,Then the hidden column will be selectedrowid来创建聚簇索引）

创建

1.在create table建表的时候添加索引

2.在create tableAfter the table is created successfully,使用alter语句添加索引

ALTER TABLE 表名 ADD INDEX 索引名（字段名）;

3.在create tableAfter the table is created successfully,使用create index语句添加索引

CREATE INDEX 索引名 ON 表名（字段名）;

删除

1.使用alter语句删除索引

ALTER TABLE 表名 DROR INDEX 索引名;

2.使用drop语句删除索引

ORDR INDEX 索引名 ON 表名;

索引的数据结构：

MYSQLThe index supports two data structures

1.B+TREE  B+树结构的索引（支持范围查询）

2.HASH  The index of the hash structure（不支持范围查询）

Evolution of data structures：

二叉树：

问题：Unable to balance itself

   

 

 

平衡二叉树-AVL树：

Left-handed algorithm：When the depth of the right leaf node-The depth of the left leaf node>1的时候,触发左旋

右旋算法：When the depth of the left leaf node-The depth of the right leaf node>1的时候,触发右旋

目的：Maintain the balance of the tree

多路平衡二叉树-B树：

树的度数：The number of elements that can be stored in each node

One-way tree：度数=1

多路树：度数=n,n>1

data of the same size,One-way trees aretall skinny tree,Multiway trees areChunky tree

The advantage of multi-way trees over single-way trees is that更少的磁盘IO就可以找到数据

 NYSQLHow the index is designed：

Each node of the tree is a disk block,MYSQL将表的ibdThe file is split into many disk blocks,The size of each disk block is uniform16KB,每次磁盘IORead a disk block of data,A disk block of data is also called a page of data

B+树和B-树的区别：

1个节点=1个磁盘块=16KB

假设一行数据data是200字节,主键id是8个字节,B树,A disk block is approximately stored8行数据,B+树,A disk block can probably be stored1000个主键id

B+树：

1.The bottommost leaf nodes form a singly linked list

2.Some elements are redundant

3.Only the bottommost leaf nodes store data,All nodes in the upper layer only store the primary keyid

B-树：

行数据dataStored with the primary key

走索引：二分查找

不走索引：全部扫描

说说索引：