LEFT JOIN Optimization

[Ram R]

Hey,

I was wondering if the WHERE condition is applied after the join or before the join. It makes sense that MySQL should do apply the condition before the JOIN but I wanted to make sure.

Here is a sample query:

SELECT user.id FROM
user
LEFT JOIN
(
SELECT blocked_id FROM blocked_users WHERE blocked_id = 34
UNION
SELECT blocked_id FROM blocked_users WHERE blocked_id = 38
) AS t
ON user.id = t.id
WHERE user.group = 40
AND t.id IS NULL;