在EXISTS查询中使用LIMIT有什么意义吗?

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问题描述

将LIMIT添加到EXISTS查询是否对性能有好处，还是MySQL自行应用该限制?

Is there any performance benefit in adding a LIMIT to an EXISTS query, or would MySQL apply the limit on its own?

示例:

IF EXISTS (
    SELECT 1
      FROM my_table
     LIMIT 1    -- can this improve performance?
)
THEN ... END IF;

推荐答案

EXISTS()的目的是只执行查询直到，直到它可以确定是否有任何行与WHERE子句匹配.也就是说，它在逻辑上与LIMIT 1做相同的事情. EXISTS在某些圈子中可能被称为semi-join.

The purpose of EXISTS() is to perform the query only until it can decide if there are any rows in that table matching the WHERE clause. That is, it logically does the same thing as LIMIT 1. EXISTS is probably called semi-join in some circles.

底线:请勿在EXISTS()内使用LIMIT 1.

附录:正如Paul所指出的，LIMIT 与以及OFFSET(或LIMIT m,n)确实具有含义.

Addenda: As Paul points out, a LIMIT with an OFFSET (or LIMIT m,n) does have meaning.

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