重修课程day44（mysql八之索引查询二）
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一 索引的创建
索引减慢了 写的操作，优化了读取的时间
index：普通索引，加速了查找的时间。
fulltext：全文索引，可以选用占用空间非常大的文本信息的字段作为索引的字段。使用fulltext时需要借助第三方的软件sphinx专用去那问搜索。
创建格式，创建表时加上索引：
create table 表名（字段1 类型 约束键，。。。。。）;
create table 表名（字段1 类型，。。。。，约束键）
mysql> create table t2(id int primary key auto_increment, -> name char(15) not null unique); Query OK, 0 rows affected (0.24 sec) mysql> desc t2; +-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | char(15) | NO | UNI | NULL | | +-------+----------+------+-----+---------+----------------+ 2 rows in set (0.01 sec) mysql> create table t3(id int, -> name char(15), -> index idx_id(id)); Query OK, 0 rows affected (0.24 sec) mysql> desc t3; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | YES | MUL | NULL | | | name | char(15) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 2 rows in set (0.01 sec)
常用的索引约束键有：primary key unique key，
普通的索引有：index
创建表后指定字段为索引字段：
create index 起名 on 表名（字段名）；
alter table 表名 add index 起名（字段名）；mysql
mysql> create table t4(id int ,
-> name char(15));
Query OK, 0 rows affected (0.25 sec)
mysql> create table t5(id int,
-> name char(15));
Query OK, 0 rows affected (0.33 sec)
mysql> desc t4;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(15) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.03 sec)
mysql> create index idx_id on t4(id);
Query OK, 0 rows affected (0.25 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t4;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | YES | MUL | NULL | |
| name | char(15) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> alter table t5 add index idx_id(id);
Query OK, 0 rows affected (0.19 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t5;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | YES | MUL | NULL | |
| name | char(15) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)
删除索引：
drop index 索引名 on 表名
删除主键：alter table 表名 drop primary key；
mysql> desc t4; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | YES | MUL | NULL | | | name | char(15) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 2 rows in set (0.01 sec) mysql> drop index idx_id on t4; Query OK, 0 rows affected (0.16 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc t4; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | char(15) | YES | | NULL | | +-------+----------+------+-----+---------+-------+
二 测试
比较符号的测试：< > = != >= <=
mysql> desc s1; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | | gender | char(6) | YES | | NULL | | | email | varchar(50) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 4 rows in set (0.01 sec) mysql> select count(*) from s1 where id=370000; +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set (0.17 sec) mysql> create index idx_id on s1(id); Query OK, 0 rows affected (1.17 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> select count(*) from s1 where id=370000; +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where id>370000; +----------+ | count(*) | +----------+ | 6220 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where id<370000; +----------+ | count(*) | +----------+ | 369999 | +----------+ 1 row in set (0.21 sec) mysql> select count(*) from s1 where id!=370000; +----------+ | count(*) | +----------+ | 376219 | +----------+ 1 row in set (0.20 sec) mysql> drop index idx_id on s1; Query OK, 0 rows affected (0.13 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> create index idx_name on s1(name); Query OK, 0 rows affected (2.19 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc s1; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | MUL | NULL | | | gender | char(6) | YES | | NULL | | | email | varchar(50) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 4 rows in set (0.01 sec) mysql> select count(*) from s1 where name=\'egon\'; +----------+ | count(*) | +----------+ | 376220 | +----------+ 1 row in set (0.25 sec) mysql> select count(*) from s1 where name=\'xxxx\'; +----------+ | count(*) | +----------+ | 0 | +----------+ 1 row in set (0.00 sec)
注意事项：插入索引过和如果添加记录会改变树形的结构。
如果一个字段的重复率过高，如果条件成立，反而会拖慢查询的效率。如果条件不成立，查询时间会非常的块
尽量选择区分度较高的字段作为索引的字段。一般是在十条记录中有一条重复。
索引的字段不能够参与计算中，因为这样条件就不会明确，所以也要从第一条记录开始计算，这样就增大了查找的范围。这样也会非常耗时间的。
逻辑符号的测试： and or not
mysql> desc s1; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | MUL | NULL | | | gender | char(6) | YES | | NULL | | | email | varchar(50) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 4 rows in set (0.01 sec) mysql> create index idx_id on s1(id); Query OK, 0 rows affected (1.69 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc s1; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | YES | MUL | NULL | | | name | varchar(20) | YES | MUL | NULL | | | gender | char(6) | YES | | NULL | | | email | varchar(50) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 4 rows in set (0.01 sec) mysql> select * from s1 where id=370000 and name=\'egon\'; +--------+------+--------+-------------------+ | id | name | gender | email | +--------+------+--------+-------------------+ | 370000 | egon | male | egon370000@oldboy | +--------+------+--------+-------------------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where name=\'egon\' and id=370000; +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where name=\'egon\' or id=370000; +----------+ | count(*) | +----------+ | 376220 | +----------+ 1 row in set (0.83 sec) mysql> select count(*) from s1 where id=370000 or name=\'egon\'; +----------+ | count(*) | +----------+ | 376220 | +----------+ 1 row in set (0.79 sec)
在and的查找中：从左到右首先找到索引区分高的索引字段，如果条件成立，在去按照索引的字段去查找，如果不成立，就不会再去查找。and前面查找的条件如果不成立，那么查找的顺序就会非常的块，如果and前面的索引字段区分度低或者查找的范围大，同样也是耗费时间的，查找效率也会很低的。
在or的查找中：从左到右一次查找记录，如果前面的条件成立，就不会查找后面的条件，如果前面的条件不成立，才会查找后面的条件。如果查找的索引字段区分度低，还是会拉低查找效率的，如果查找的索引字段区分度高的话，那就要看查找的范围，如果查找的范围过大，效率还是不会提升的，如果明确一个条件，或着范围小，这样就会提升查找的效率。
范围的比较：in between like
mysql> select count(*) from s1 where id in (213,54343,354544); +----------+ | count(*) | +----------+ | 3 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where id between 100 and 3000; +----------+ | count(*) | +----------+ | 2901 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where id between 100 and 370000; +----------+ | count(*) | +----------+ | 369901 | +----------+ 1 row in set (0.19 sec) mysql> select count(*) from s1 where id not in (1232,3423,24324); +----------+ | count(*) | +----------+ | 376217 | +----------+ 1 row in set (0.20 sec)
联合索引的测试：and or
联合索引就是将需要判断的字段联合起来创建一个索引。联合索引都是后面的字段联合第一个的。
mysql> desc s1; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | | gender | char(6) | YES | | NULL | | | email | varchar(50) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 4 rows in set (0.01 sec) mysql> create index idx_xxx on s1(name,gender,id,email); Query OK, 0 rows affected (3.19 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc s1; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | MUL | NULL | | | gender | char(6) | YES | | NULL | | | email | varchar(50) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 4 rows in set (0.01 sec) mysql> select count(*) from s1 where name=\'egon\' and gender=\'male\' and id>3000 and email=\'xxx\'; +----------+ | count(*) | +----------+ | 0 | +----------+ 1 row in set (0.49 sec) mysql> select count(*) from s1 where name=\'egon\' and gender=\'male\' and email=\'xxx\' and id>3000; +----------+ | count(*) | +----------+ | 0 | +----------+ 1 row in set (0.49 sec) mysql> select count(*) from s1 where name=\'egon\' and gender=\'male\' and email=\'xxx\' and id=3000; +----------+ | count(*) | +----------+ | 0 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where name=\'egon\' and gender=\'male\' and id=3000; +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where name=\'egon\' and id=370000 and gender=\'male\'; +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from s1 where name=\'egon\' and gender=\'male\' and email=\'xxx\'; +----------+ | count(*) | +----------+ | 0 | +----------+ 1 row in set (0.46 sec) mysql> select count(*) from s1 where name=\'egon\' and id=370000 and email=\'xxx\'; +----------+ | count(*) | +----------+ | 0 | +----------+ 1 row in set (0.30 sec)
注意：如果联合索引的字段条件没有连续出现的话也会拖慢速度的。
什么叫做最左前缀：联合索引的查找顺序是从左到右依次查找，如果是and，查找出区分度高的索引字段先执行，如果条件成立在执行其他的条件，如果不成立，就不会执行其他的条件；如果是or，从左到右依次执行，如果条件成立就不会在去执行其他的索引条件。
注意事项：应该将明确的条件字段放在右范围的条件字段的前面。
排序字段必须是索引字段。
对于联合索引，只要有第一个联合索引的字段，就会起作用，如果没有，就不会有效果。
覆盖索引只要有联合索引的第一个字段就可以使用
合并索引：创建多个单列索引，他们可以合并的使用，也可以单个使用。
详细资料访问：http://www.cnblogs.com/linhaifeng/articles/7274563.html#_label7
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