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MySQL索引相关操作小结

备注:测试数据库版本为MySQL 8.0

这个blog我们来聊聊MySQL 索引相关操作

文章目录

一.创建索引

语法:

CREATE [UNIQUE | FULLTEXT | SPATIAL] INDEX index_name
    [index_type]
    ON tbl_name (key_part,...)
    [index_option]
    [algorithm_option | lock_option] ...

key_part: {col_name [(length)] | (expr)} [ASC | DESC]

index_option: {
    KEY_BLOCK_SIZE [=] value
  | index_type
  | WITH PARSER parser_name
  | COMMENT 'string'
  | {VISIBLE | INVISIBLE}
}

index_type:
    USING {BTREE | HASH}

algorithm_option:
    ALGORITHM [=] {DEFAULT | INPLACE | COPY}

lock_option:
    LOCK [=] {DEFAULT | NONE | SHARED | EXCLUSIVE}

1.1 前缀索引

前缀索引可以使用在 CHAR, VARCHAR, BINARY, and VARBINARY 类型的列上
当列的类型为 BLOB和TEXT的时候，必须InnoDB, MyISAM, and BLACKHOLE的表，而且必须指定长度

当要索引的列字符很多时 索引则会很大且变慢
( 可以只索引列开始的部分字符串 节约索引空间 从而提高索引效率 )

原则： 降低重复的索引值

通过前缀索引找到对应的行，然后通过where过滤掉值不同的行，可以找到准确的数据

create table t1(id int,name varchar(100));
insert into t1 (id,name) values (1,'abcdefghijklmn');
insert into t1 (id,name) values (1,'abcdefghijklmnopq');
-- 创建前缀索引
create index i_t1_partname on t1(name(10));

测试记录

mysql> create table t1(id int,name varchar(100));
Query OK, 0 rows affected (0.03 sec)

mysql>
mysql> insert into t1 (id,name) values (1,'abcdefghijklmn');
Query OK, 1 row affected (0.01 sec)
mysql> insert into t1 (id,name) values (1,'abcdefghijklmnopq');
Query OK, 1 row affected (0.01 sec)
mysql> create index i_t1_partname on t1(name(10));
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> select * from t1;
+------+-------------------+
| id   | name              |
+------+-------------------+
|    1 | abcdefghijklmn    |
|    1 | abcdefghijklmnopq |
+------+-------------------+
2 rows in set (0.00 sec)
mysql>
mysql> select * from t1 where name = 'abcdefghijklmn';
+------+----------------+
| id   | name           |
+------+----------------+
|    1 | abcdefghijklmn |
+------+----------------+
1 row in set (0.00 sec)

text列前缀索引

create table t2(id int,name text);
insert into t2 values(1,'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa');
insert into t2 values(1,'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb');
create index i_t2_name on t2(name);
create index i_t2_name on t2(name(10));

测试记录

mysql>
mysql> create table t2(id int,name text);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into t2 values(1,'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa');
Query OK, 1 row affected (0.01 sec)

mysql> insert into t2 values(1,'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb');
Query OK, 1 row affected (0.01 sec)

mysql>
mysql> create index i_t2_name on t2(name);
ERROR 1170 (42000): BLOB/TEXT column 'name' used in key specification without a key length
mysql>
mysql> create index i_t2_name on t2(name(10));
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

1.2 函数索引

create table t3(id int,name varchar(100),index idx1 ((substring(name, 1, 10))));
show create table t3\G
insert into t3 values (1,'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa');
insert into t3 values (1,'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb');
insert into t3 values (1,'ccccccccccccccccccccccccccccccccccccc');
explain select * from t3 where substring(name,1,10) = 'aaaaaaaaaa';

测试记录:

mysql> create table t3(id int,name varchar(100),index idx1 ((substring(name, 1, 10))));
Query OK, 0 rows affected (0.03 sec)

mysql> show create table t3\G
*************************** 1. row ***************************
       Table: t3
Create Table: CREATE TABLE `t3` (
  `id` int DEFAULT NULL,
  `name` varchar(100) DEFAULT NULL,
  KEY `idx1` ((substr(`name`,1,10)))
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql>
mysql> insert into t3 values (1,'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa');
Query OK, 1 row affected (0.01 sec)

mysql> insert into t3 values (1,'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb');
Query OK, 1 row affected (0.01 sec)

mysql> insert into t3 values (1,'ccccccccccccccccccccccccccccccccccccc');
Query OK, 1 row affected (0.01 sec)
mysql>
-- where后面带 substring是可以用到索引的
mysql> explain select * from t3 where substring(name,1,10) = 'aaaaaaaaaa';
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | t3    | NULL       | ref  | idx1          | idx1 | 33      | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

1.3 唯一索引

create table t5(id int,idcard varchar(50),unique index idx2 (idcard));
show create table t5\G
insert into t5(id,idcard) values (1,'123');
insert into t5(id,idcard) values (2,'123');

测试记录:

mysql>
mysql> create table t5(id int,idcard varchar(50),unique index idx2 (idcard));
Query OK, 0 rows affected (0.03 sec)

mysql> show create table t5\G
*************************** 1. row ***************************
       Table: t5
Create Table: CREATE TABLE `t5` (
  `id` int DEFAULT NULL,
  `idcard` varchar(50) DEFAULT NULL,
  UNIQUE KEY `idx2` (`idcard`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.01 sec)

mysql>
mysql> insert into t5(id,idcard) values (1,'123');
Query OK, 1 row affected (0.01 sec)

mysql> insert into t5(id,idcard) values (2,'123');
ERROR 1062 (23000): Duplicate entry '123' for key 't5.idx2'
mysql>

1.4 全文索引

全文索引只适用于 InnoDB、MyISAM存储引擎
也只能创建在CHAR、VARCHAR、TEXT列上

MySQL的全文索引目前而言还不是太适合，有类似需求的基本上的都是elsticsearch数据库。

1.5 多值索引

用于给json列创建索引，应用场景也不多，此处略过。

1.6 空间索引

数据库可以存储地理位置信息，就是空间数据
MySQL存储空间数据不多见，此处略过

1.7 索引相关选项

1.7.1 KEY_BLOCK_SIZE

MyISAM使用，InnoDB存储引擎已不适用

1.7.2 index_type

各个存储引擎支持的索引类型

存储引擎支持的索引类别
InnoDBBTREE
MyISAMBTREE
MEMORY/HEAPHASH, BTREE
NDBHASH, BTREE
1.7.3 COMMENT ‘string’

索引的注释语句

1.7.4 VISIBLE, INVISIBLE

可见和不可见索引

二.删除索引

语法:

DROP INDEX index_name ON tbl_name
    [algorithm_option | lock_option] ...

algorithm_option:
    ALGORITHM [=] {DEFAULT | INPLACE | COPY}

lock_option:
    LOCK [=] {DEFAULT | NONE | SHARED | EXCLUSIVE}

Oracle 里面，索引和表是同一个命名空间，索引名是不存在重复
MySQL里面不同的表，可以有相同的索引名

show create table t5\G
drop index idx2 on t5;
create index idx2 on t5(idcard);
create table t6(id int,idcard varchar(50),unique index idx2 (idcard));
show create table t6\G

测试记录:

mysql> show create table t5\G
*************************** 1. row ***************************
       Table: t5
Create Table: CREATE TABLE `t5` (
  `id` int DEFAULT NULL,
  `idcard` varchar(50) DEFAULT NULL,
  UNIQUE KEY `idx2` (`idcard`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql>
mysql> drop index idx2 on t5;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql>
mysql> create index idx2 on t5(idcard);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>
mysql> create table t6(id int,idcard varchar(50),unique index idx2 (idcard));
Query OK, 0 rows affected (0.06 sec)

mysql>
mysql> show create table t6\G
*************************** 1. row ***************************
       Table: t6
Create Table: CREATE TABLE `t6` (
  `id` int DEFAULT NULL,
  `idcard` varchar(50) DEFAULT NULL,
  UNIQUE KEY `idx2` (`idcard`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql>

悦读

道可道，非常道；名可名，非常名。 无名，天地之始，有名，万物之母。 故常无欲，以观其妙，常有欲，以观其徼。 此两者，同出而异名，同谓之玄，玄之又玄，众妙之门。

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