SQL获取行数优化 [英] SQL getting row count optimization

问题描述

可能重复：

）之间有什么区别？

Count（）vs Count（1）

Possible Duplicate:
In SQL, what's the difference between count(column) and count()?
Count() vs Count(1)

我有一张大桌子，上面放着很长的文字，例如电子邮件内容或新闻。我的问题
计数表行的性能是否有所不同：

I have big tables which keep long texts for example email content or news. And my question is there any difference for performance for counting table rows :

SELECT COUNT(*) FROM table_name

SELECT COUNT(t.id) FROM table_name as t

哪个是更好？还是后者将被查询优化器优化？

Which one is better? or the latter one will be optimized by query optimizer? Is there any documentation regarding this?

推荐答案

它们是不同的。

COUNT（*）将检索所有值（甚至 NULL 值计数）。
COUNT（t.id）不计算 NULL 值。

COUNT(*) will retrieve all values (even the NULL values count). COUNT(t.id) doesn't count the NULL values.

就性能而言，它们是相同的（查询优化器很聪明）。

In terms of performance, they are the same (the query optimizer is smart).

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