内连接和in（）子句的性能在哪里？

Question

I can get same result for these queries, but which one is the fastest, and most efficient?

where in() or inner join?

SELECT `stats`.`userid`,`stats`.`sumpoint` 
FROM  `stats` 
INNER JOIN users
ON `stats`.`userid` = `users`.`userid` 
WHERE `users`.`nick` =  '$nick'

ORDER BY `statoylar`.`sumpoint` DESC  limit 0,10

and

SELECT `stats`.`userid`,`stats`.`sumpoint` 
FROM  `stats` 
WHERE userid
IN (
SELECT userid
FROM  `users` 
WHERE  `users`.`nick` =  '$nick'
)
ORDER BY `stats`.`sumpoint` DESC  limit 0,10

Answer 1

Depends on your SQL engine. Newer SQL systems that have reasonable query optimizers will most likely rewrite both queries to the same plan. Typically, a sub-query (your second query) is rewritten using a join (the first query).

In simple SQL engines that may not have great query optimizers, the join should be faster because they may run sub-queries into a temporary in-memory table before running the outer query.

In some SQL engines that have limited memory footprint, however, the sub-query may be faster because it doesn't require joining -- which produces more data.

So, in summary, it depends.

Answer 2

to check the performance execute both Query with EXPLAIN SELECT .... AFAIK, INNER JOIN is faster than IN
btw what is your type of table engine MYISAM or INNODB

Answer 3

also there is another option, EXISTS. I'm a tsql guy so....

SELECT s.[userid], s.[sumpoint] 
FROM stats AS s
WHERE
     EXISTS (
     SELECT 1
     FROM users AS u
     WHERE
         u.[userID] = s.[userID]
         AND u.[nick] = '$nick'
     )
ORDER BY s.[sumpoint] DESC

I think EXISTS is available in most engines. It's generally pretty fast.

IN sql server at least (2005+) there is no performance difference at all between IN and EXISTS for cases where the column in question is not NULLABLE.

probably irrelevant but hey.....