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  • mysql的explain

    explain 一般用于分析sql.  
    如下

    [SQL] 纯文本查看 复制代码
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    drop table if exists test1;
    CREATE TABLE test1 (
        id INT NOT NULL primary key  auto_increment, -- 自动递增
        username varchar(5) not null -- 用户名
    )
    ENGINE=innodb ;
     
    insert into test1 (username) values (concat('test',rand()));
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;
    insert into test1 (username)   select username from test1;


    先创建一部分数据. 然后执行

    [SQL] 纯文本查看 复制代码
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    explain select * from test1;


    结果

    explain select username from test1;
    +----+-------------+-------+------+---------------+------+---------+------+---------+-------+
    | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra |
    +----+-------------+-------+------+---------------+------+---------+------+---------+-------+
    |  1 | SIMPLE      | test1 | ALL  | NULL          | NULL | NULL    | NULL | 4187248 | NULL  |
    +----+-------------+-------+------+---------------+------+---------+------+---------+-------+
    1 row in set (0.00 sec)


    再执行 

    [SQL] 纯文本查看 复制代码
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    explain select username from test1 where username = 'test';


    +----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
    | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
    +----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
    |  1 | SIMPLE      | test1 | ALL  | NULL          | NULL | NULL    | NULL | 4187248 | Using where |
    +----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
    1 row in set (0.00 sec)


    接下来再给表的username创建一个索引

    [SQL] 纯文本查看 复制代码
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    ALTER TABLE `test`.`test1` ADD INDEX `i_name` (`username`) comment '';

       过程 会比较 卡.需要等待几秒到几十秒.取决于机器. 也可以少插入几条数据. 然后再执行刚才的两条语句

    mysql> explain select username from test1 where username = 'test';
    +----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
    | id | select_type | table | type | possible_keys | key    | key_len | ref   | rows | Extra                    |
    +----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
    |  1 | SIMPLE      | test1 | ref  | i_name        | i_name | 7       | const |    1 | Using where; Using index |
    +----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
    1 row in set (0.00 sec)

    mysql> explain select username from test1;
    +----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
    | id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows    | Extra       |
    +----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
    |  1 | SIMPLE      | test1 | index | NULL          | i_name | 7       | NULL | 4187248 | Using index |
    +----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
    1 row in set (0.00 sec)


    发现 type 变了   key也变了.  extra也不一样了. 对比一下就了解情况

    其中,

    type=const表示通过索引一次就找到了;

    key=primary的话,表示使用了主键;

    type=all,表示为全表扫描;

    key=null表示没用到索引。
    type=ref,因为这时认为是多个匹配行,在联合查询中,一般为REF。

    经过分析后就可以判断出哪些字段需要建 索引,哪些条件可以优化等. 以及条数等



    system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > A

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  • 原文地址:https://www.cnblogs.com/ghjbk/p/6707225.html
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