如何解释这个警告

Question

我正在使用MySQL 5.7.19并在以下脚本上运行explain之后：

explain select a.seller_id AS seller_id, 
       a.seller_name AS seller_name, 
       b.user_name AS user_name, 
       c.state AS state
from a
join b
join c
where((b.seller_name = a.seller_name) and
      (c.user_id = 17) and(b.user_id = 17) and
      (a.gmt_create between ((now() - interval 600 minute)) and ((now() + interval 600 minute)))) 
order by a.gmt_create;

以下是我收到的警告信息：

将其置于人类可读的形式：

select a.seller_id AS seller_id, 
           a.seller_name AS seller_name, 
           b.user_name AS user_name, 
           c.state AS state
    from a
    join b
    join c
    where((b.seller_name = a.seller_name) and
          (c.user_id = 17) and(b.user_id = 17) and
          (a.gmt_create between <cache>((now() - interval 600 minute)) and <cache>((now() + interval 600 minute)))) 
    order by a.gmt_create;

<cache>标签意味着什么？

Answer 1

引用manual：

  

  
• <cache>
  

     

表达式（例如标量子查询）执行一次，然后执行   结果值保存在内存中供以后使用。为了结果   由多个值组成，可以创建临时表   你会看到<temporary table>。

这意味着，now() - interval 600 minute的计算只进行一次，结果将用于where子句检查的每一行。