MySQL最大值 - 时间和频率

Question

If I have a query like

SELECT (MAX(b.A)/a.A) AS c
FROM a
INNER JOIN b
ON a.b_id = b.id
GROUP by a.id

does MySQL evaluate the value for "MAX(bA)" for every row or only once?

It's just of interest to me if there is room for performance improvement or not.

Thanks!

UPDATE

OK let's move on to a real world example: I want to calculate the proportional value of a users likes compared to max-user-likes.

The query to only read the max value of users.likes (which is indexed) takes 0.0003

SELECT MAX(likes) 
FROM users

So I now know the value of max-user-likes, let's say it's 10000 so I could query like this which takes 0.0007s:

SELECT (users.likes/10000) AS weight 
FROM posts
INNER JOIN users ON posts.author_id = users.id 

So one would expect to have both queries together to be something like 0.0003 + 0.0007s, but it takes 0.3s:

SELECT (users.likes/(SELECT MAX(likes) FROM users)) AS weight 
FROM posts
INNER JOIN users ON posts.author_id = users.id 

So something seems still wrong with my database - any suggestions?

Answer 1

Since you have no GROUP BY clause, the result will only have one row and you can't know from which row the value of aA will be. The value of MAX(bA) will be only evaluated once.

When you have a GROUP BY clause, MAX(bA) will be evaluated for every group .

Answer 2

In general, the expression within an aggregation function is evaluated and checked for NULL for each row. There certainly can be a optimization for MIN and MAX in case of walking through an index, but I doubt that.

BTW, you can easily check this, when you execute MAX(id) on a large table. You will see that the execution time is the same as for COUNT(id) (and might be much more than COUNT(*) depending on the engine).