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  • MySQL中如何分析查询语句

    Oracle中有explain for,mysql中也有同样的功能,那便是explain,举例如下:

    mysql> explain select (case (select count(*) from stock) when 0 then 0 else max(id)+1 end) from stock;
    +----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
    | id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                        |
    +----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
    |  1 | PRIMARY     | NULL  | NULL  | NULL          | NULL    | NULL    | NULL | NULL | Select tables optimized away |
    |  2 | SUBQUERY    | stock | index | NULL          | PRIMARY | 4       | NULL | 3696 | Using index                  |
    +----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
    2 rows in set (0.00 sec)

    desc也是同样的功能:

    mysql> desc select (case (select count(*) from stock) when 0 then 0 else max(id)+1 end) from stock;
    +----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
    | id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                        |
    +----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
    |  1 | PRIMARY     | NULL  | NULL  | NULL          | NULL    | NULL    | NULL | NULL | Select tables optimized away |
    |  2 | SUBQUERY    | stock | index | NULL          | PRIMARY | 4       | NULL | 3696 | Using index                  |
    +----+-------------+-------+-------+---------------+---------+---------+------+------+------------------------------+
    2 rows in set (0.00 sec)

    其中rows可以当cost那样看。

    --2020年5月2日--

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  • 原文地址:https://www.cnblogs.com/heyang78/p/12817392.html
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