我应该不算(*)吗? [英] Should I COUNT(*) or not?

问题描述

我知道执行这样的查询通常不是一个好主意:

I know it's generally a bad idea to do queries like this:

SELECT * FROM `group_relations`

但是当我只想计数时，我应该进行此查询，因为这允许更改表，但仍然产生相同的结果.

But when I just want the count, should I go for this query since that allows the table to change but still yields the same results.

SELECT COUNT(*) FROM `group_relations`

或更具体的

SELECT COUNT(`group_id`) FROM `group_relations`

我觉得后者可能会更快，但是还有其他需要考虑的东西吗?

I have a feeling the latter could potentially be faster, but are there any other things to consider?

更新:在这种情况下，我使用的是InnoDB，对不起，因为它不够具体.

Update: I am using InnoDB in this case, sorry for not being more specific.

推荐答案

如果所讨论的列不是NULL，则两个查询都是等效的.当group_id包含空值时，

If the column in question is NOT NULL, both of your queries are equivalent. When group_id contains null values,

select count(*)

将计算所有行，而

select count(group_id)

仅计算group_id不为null的行.

will only count the rows where group_id is not null.

此外，某些数据库系统(例如MySQL)在请求count(*)时进行了优化，从而使此类查询比特定查询快一点.

Also, some database systems, like MySQL employ an optimization when you ask for count(*) which makes such queries a bit faster than the specific one.

就个人而言，当我计数时，我正在做count(*)以便安全地使用空值.

Personally, when just counting, I'm doing count(*) to be on the safe side with the nulls.

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