mysql - inner join and where in() clause performance?

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.