索引
一、索引的分類
1.根據算法分類
1）主鍵索引
#1.建表時創建
create table test(id int primary key);
create table test(id int,primary key(id));
#2.添加主鍵索引
alter table test add primary key pri_key(id);
2）唯一建索引
#1.建表時創建
create table test(id int unique key);
#2.添加唯一建索引
alter table test add unique key uni_key(id);
3）普通索引
#1.添加普通索引
alter table test add index ljp_key(id);
4）全文索引
5）查看索引
#1.方式一：
mysql> show index from test10;
#2.方式二：
mysql> desc test10;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | UNI | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)
PRI：主鍵索引
UNI：唯一建索引
MUL：普通索引
6）刪除索引
mysql> alter table test drop index index_key;
2.根據配置方法分類
1）注意事項
1.創建索引時會將數據重新進行排序
2.創建索引會占用磁盤空間，所以索引不是越多越好
3.在同一列上避免創建多種索引
4.避免在數據很長的字段上創建索引，如果要創建就創建前綴索引
2）前綴索引
#根據前四個字符創建前綴索引
mysql> alter table test add index index_key(name(4));
3）聯合索引
mysql> create database xiangqing;
mysql> create table xiangqin(id int,name varchar(20),gender enum('m','f'),age tinyint,money int,height int,weight int,looks tinyint);
mysql> insert xiangqin values(1,'qiudao','m',38,-200000,120,130,'10'),(2,'dilireba','f',18,400000,180,100,'60'),(3,'cxk','m',28,100000,170,120,'440'),(4,'fbb','f',18,1000000,165,85,'90');
#創建聯合索引
mysql> alter table xiangqin add index lh_key(money,gender,age,looks);
#聯合索引使用三種情況
1.部分走索引 money,gender,age
2.全部走索引 money,gender,age,looks
3.不走索引 gender,age
二、explain的使用
1.explain語法
explain + DQL語句
mysql> explain select * from city where countrycode ='CHN' or countrycode ='USA';
#查詢中國和美國的數據
mysql> select * from city where countrycode ='CHN' or countrycode ='USA';
mysql> select * from city where countrycode in ('CHN','USA');
mysql> select * from city where countrycode = 'CHN' union all select * from city where countrycode = 'USA';
Extra（擴展）
Using temporary 使用group by大概率出現
Using filesort 使用了order by大概率出現
Using join buffer 使用join on大概率出現
2.擴展group by
#一般與聚合索引一起使用
#建表
mysql> create table jixiao(id int,name varchar(20) charset utf8,jixiao int,product varchar(10) charset utf8);
Query OK, 0 rows affected (0.03 sec)
#插入數據
mysql> insert jixiao values(1,'qiudao','1000000','房地產'),(2,'niulei','10000','房地產'),(3,'lijianpeng','100000','汽車'),(4,'qiandao','200000',' 汽車');
#查詢不同行業績效最高的人
mysql> select name,sum(jixiao),product from jixiao group by product;
+------------+-------------+-----------+
| name | sum(jixiao) | product |
+------------+-------------+-----------+
| qiudao | 1010000 | 房地產 |
| lijianpeng | 300000 | 汽車 |
+------------+-------------+-----------+
2 rows in set (0.00 sec)
#查詢房地產行業績效最高的人
mysql> select name,sum(jixiao),product from jixiao group by product having product='房地產';
+--------+-------------+-----------+
| name | sum(jixiao) | product |
+--------+-------------+-----------+
| qiudao | 1010000 | 房地產 |
+--------+-------------+-----------+
1 row in set (0.00 sec)
3.查詢數據的方式
1）全表掃描
#1.什么是全表掃描
查詢數據時type類型為ALL
#2.什么情況全表掃描
1）查詢數據庫所有數據
mysql> explain select * from country
2）沒有走索引
沒設置索引
索引損壞
2）索引掃描
1.index #全索引掃描
mysql> explain select Name from city;
2.range #范圍查詢
mysql> explain select * from city where countrycode ='CHN' or countrycode ='USA';
#有限制查詢到的數據在總數據的20%以內，超過則走全文掃描，所以在查詢是可以使用limit限制
mysql> explain select * from city where countrycode != 'CHN' limit 500;
3.ref #精確查詢
mysql> explain select * from city where countrycode ='CHN';
4.eq_ref #使用join on時偶爾會出現
5.const #查詢條件是唯一索引或主鍵索引
mysql> explain select * from city where id=1;
6.system #查詢級別與const一樣，當數據很少時為該級別
7.null #不需要讀取數據，只需要獲取最大值或者最小值
mysql> explain select max(population) from city;
三、索引的建立
1.索引的建立原則
1.能創建唯一索引就創建唯一索引
2.為經常需要排序、分組和聯合操作的字段建立索引
3.為常作為查詢條件的字段建立索引
如果某個字段經常用來做查詢條件，那么該字段的查詢速度會影響整個表的查詢速度。
因此，為這樣的字段建立索引，可以提高整個表的查詢速度。
4.盡量使用前綴來索引
如果索引字段的值很長，最好使用值的前綴來索引。
例如，TEXT和BLOG類型的字段，進行全文檢索，會很浪費時間。如果只檢索字段的前面的若干個字符，這樣可以提高檢索速度。
5.限制索引的數目
索引的數目不是越多越好。每個索引都需要占用磁盤空間，索引越多，需要的磁盤空間就越大。
修改表時，對索引的重構和更新很麻煩。越多的索引，會使更新表變得很浪費時間。
6.刪除不再使用或者很少使用的索引
表中的數據被大量更新，或者數據的使用方式被改變后，原有的一些索引可能不再需要。數據庫管理員應當定期找出這些索引，將它們刪除，從而減少索引對更新操作的影響。
2.總結什么時候不走索引
1）沒有查詢條件，或者查詢條件沒有索引
#沒有查詢條件
mysql> explain select * from city;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | city | ALL | NULL | NULL | NULL | NULL | 4188 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)
#查詢條件沒有索引
mysql> explain select District from city;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | city | ALL | NULL | NULL | NULL | NULL | 4188 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)
2）查詢的結果占總數據的20%左右
#占總數據的18%，沒走索引
mysql> explain select * from city where population > 400000;
#占總數據的15%，走了索引
mysql> explain select * from city where population > 450000;
#如果數據量查詢就是表中大部分數據，可以用limit做限制
mysql> explain select * from city where population > 400000 limit 100;
3）索引損壞
4）查詢條件帶了特使符號(+，-)
#在=號左側有特殊符號，不走索引
mysql> explain select * from city where id-1=1;
#在=號右側有特殊符號，走索引
mysql> explain select * from city where id=3-1;
5）隱式轉換
#建表
mysql> create table test (id int ,name varchar(20),telnum varchar(10));
Query OK, 0 rows affected (0.04 sec)
#插入數據
mysql> insert into test values(1,'zs','110'),(2,'l4',120),(3,'w5',119),(4,'z4',112);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
#建立索引
mysql> desc phonenum;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| phone | varchar(10) | YES | UNI | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
#查詢語句級別全文掃描
mysql> explain select * from phonenum where phone=6666666;
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | phonenum | ALL | uni_key | NULL | NULL | NULL | 3 | Using where |
+----+-------------+----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
#當給字符加上引號，查詢為索引掃描
mysql> explain select * from phonenum where phone='6666666';
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | phonenum | const | uni_key | uni_key | 13 | const | 1 | NULL |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
6）like "%_" 百分號在最前面不走
#走range索引掃描
EXPLAIN SELECT * FROM teltab WHERE telnum LIKE '31%';
#不走索引
EXPLAIN SELECT * FROM teltab WHERE telnum LIKE '%110';
7）聯合索引查詢不按照順序有可能不走索引
%linux%類的搜索需求，可以使用Elasticsearch -------> ELK
單獨引用聯合索引里非第一位置的索引列
CREATE TABLE t1 (id INT,NAME VARCHAR(20),age INT ,sex ENUM('m','f'),money INT);
ALTER TABLE t1 ADD INDEX t1_idx(money,age,sex);
DESC t1
SHOW INDEX FROM t1
#走索引的情況測試
EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE money=30 AND age=30 AND sex='m';
#部分走索引
EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE money=30 AND age=30;
EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE money=30 AND sex='m';
#不走索引
EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE age=20
EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE age=30 AND sex='m';
EXPLAIN SELECT NAME,age,sex,money FROM t1 WHERE sex='m';