Question

I’ve got a bit of a nasty query with several subselects that are really slowing it down. I’m already caching the query, but the results of it changes often and the query results are meant to be shown on a high traffic page.

SELECT user_id, user_id AS uid, (SELECT correct_words 
                                FROM score 
                                WHERE user_id = `uid` 
                                ORDER BY correct_words DESC, incorrect_words ASC 
                                LIMIT 0, 1) AS correct_words,
                                (SELECT incorrect_words 
                                FROM score 
                                WHERE user_id = `uid` 
                                ORDER BY correct_words DESC, incorrect_words ASC 
                                LIMIT 0, 1) AS incorrect_words
FROM score
WHERE user_id > 0
AND DATE(date_tested) = DATE(NOW())
GROUP BY user_id
ORDER BY correct_words DESC,incorrect_words ASC
LIMIT 0,7

The goal of the query is to pick out the top score for users for that day, but only show the highest scoring instance of that user instead of all of their scores (So, for instance, if one user actually had 4 of the top 10 scores for that day, I only want to show that user’s top score and remove the rest)

Try as I might, I’ve yet to replicate the results of this query any other way. Right now its average run time is about 2 seconds, but I’m afraid that might increase greatly as the table gets bigger.

Any thoughts?

Answer 1

try this:

The subquery basically returns the resultset of all the scores in the right order, and the outer query greps out the first occurence. When grouping in MySQL, columns that are not grouped on return the equivalent to FIRST(column): the value of the first occurence.

SELECT user_id, correct_words, incorrect_words
FROM
( SELECT user_id, correct_words, incorrect_words
  FROM score
  WHERE user_id>0
  AND DATE(date_tested)=DATE(NOW())
  ORDER BY correct_words DESC,incorrect_words ASC
)
GROUP BY user_id
LIMIT 0,7