mysql - 如何解释这个警告

Question

我正在使用 MySQL 5.7.19，在我运行 explain 之后在以下脚本中:

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;

这是我收到的警告信息:

将其转化为人类可读的形式:

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> 是什么意思？标签是什么意思？？

Answer 1

引用manual :

• <cache>

The expression (such as a scalar subquery) is executed once and the resulting value is saved in memory for later use. For results consisting of multiple values, a temporary table may be created and you will see <temporary table> instead.

这意味着，您计算的 now() - interval 600 minute仅执行一次，结果用于您的 where 子句检查的每一行。