Database Design/SQL Optimisation: WHERE <id> NOT IN (thousands of IDs) - mysql

Question

I'v been asked to add functionality to an application that lets users vote between two options: A and B. The table for those questions is very basic:
QUESTIONS
question_id (PK)
option_id_1(FK)
option_id_2(FK)
urgent (boolean)
Each time a user votes, that the user has voted is stored in an equally simple table:
USER VOTES
vote_id (PK)
user_id (FK)
question_id (FK)
The algorithm for selecting which question appears when a user requests a new one is complex, but for our purposes we can assume it's random. So, the issue?
Each user will be voting on many questions. Likely hundreds, and possibly thousands. I need to ensure no user is presented with a question they've already voted on, and the only way I can think to do that will, I'm guessing, pound the server into oblivion. Specifically, something like:
SELECT * from questions WHERE question_id NOT in (SELECT question_id from user_votes WHERE user_id = <user_id>) ORDER BY RAND() LIMIT 1.
[Note: RAND() is not actually in the query - it's just there as a substitute for a slightly complex (order_by).]
So, keeping in mind that many users could well have voted on hundreds if not thousands of questions, and that it's not possible to present the questions in a set order...any ideas on how to exclude voted-on questions without beating my server into the ground?
All advice appreciated - many thanks.

Answer 1

JOIN operator perform much better than nested queries in MySQL(that might have changed with latest MySQL releases but if you are experiencing performance problems than i guess my statement still holds)
what you could do is simply left join votes onto questions and only pick those records where none votes were joined(nobody voted):
SELECT *
FROM questions q
LEFT JOIN user_votes uv ON
uv.question_id = q.question_id AND
uv.user_id = '<user_id>'
WHERE vote_id IS NULL

Answer 2

RAND() is nasty however this may mitigate the problem while giving you the results you need. Seeing as you have mentioned that the RAND() is an example, I can't really provide more specific suggestions than that below however replacing the ORDER BY should work just fine.
The more you are able to limit the number of rows in the inner query, the faster the entire query will perform.
SELECT
q.*
FROM (
-- First get the questions which have not been answered
SELECT
questions.*
FROM questions
LEFT JOIN user_votes
ON user_votes.question_id = questions.question_id
AND user_votes.user_id = <user_id>
WHERE user_votes.user_id IS NULL
) q
-- Now get a random 1. I hate RAND().
ORDER BY RAND()
LIMIT 1