No, this won't be efficient with millions of rows.
You'll have to create a supporting aggregate table which would store votes per item:
CREATE TABLE item_votes
(
item_id INT NOT NULL PRIMARY KEY,
votes UNSIGNED INT NOT NULL,
upvotes UNSIGNED INT NOT NULL,
downvotes UNSIGNED INT NOT NULL,
KEY (votes),
KEY (upvotes),
KEY (downvotes)
)
and update it each time a vote is cast:
INSERT
INTO item_votes (item_id, votes, upvotes, downvotes)
VALUES (
$item_id,
CASE WHEN $upvote THEN 1 ELSE -1 END,
CASE WHEN $upvote THEN 1 ELSE 0 END,
CASE WHEN $upvote THEN 0 ELSE 1 END
)
ON DUPLICATE KEY
UPDATE
SET votes = votes + VALUES(upvotes) - VALUES(downvotes),
upvotes = upvotes + VALUES(upvotes),
downvotes = downvotes + VALUES(downvotes)
then select top 10 votes:
SELECT *
FROM item_votes
ORDER BY
votes DESC, item_id DESC
LIMIT 10
efficiently using an index.