Question

SELECT i.*, i.id IN (   SELECT id   FROM w    WHERE w.status='active') AS wish  FROM i INNER JOIN r ON i.id=r.id WHERE r.member_id=1 && r.status='active'  ORDER BY wish DESC  LIMIT 0,50 

That’s a query that I’m trying to run. It doesn’t scale well, and I’m wondering if someone here can tell me where I could improve things. I don’t join w to r and i because I need to show rows from i that are unrepresented in w. I tried a left join, but it didn’t perform too well. This is better, but not ideal yet. All three tables are very large. All three are indexed on the fields I’m joining and selecting on.

Any comments, pointers, or constructive criticisms would be greatly appreciated.

EDIT Addition:

I should have put this in my original question. It’s the EXPLAIN as return from SQLYog.

id|select_type       |table|type          |possible_keys|key      |key_len|ref  |rows|Extra|   1 |PRIMARY           |r    |ref           |member_id,id |member_id|3      |const|3120|Using where; Using temporary; Using filesort   1 |PRIMARY           |i    |eq_ref        |id           |id       |8      |r.id |1   |   2 |DEPENDENT SUBQUERY|w    |index_subquery|id,status    |id       |8      |func |8   |Using where 

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EDIT le dorfier – more comments …

I should mention that the key for w is (member_id, id). So each id can exist multiple times in w, and I only want to know if it exists.

Answer 1

WHERE x IN () is identical to an INNER JOIN to a SELECT DISTINCT subquery, and in general, a join to a subquery will typically perform better if the optimizer doesn’t turn the IN into a JOIN – which it should:

SELECT i.* FROM i INNER JOIN (     SELECT DISTINCT id     FROM w      WHERE w.status = 'active' ) AS wish      ON i.id = wish.id INNER JOIN r     ON i.id = r.id WHERE r.member_id = 1 && r.status = 'active'  ORDER BY wish.id DESC  LIMIT 0,50 

Which, would probably be equivalent to this if you don’t need the DISTINCT:

SELECT i.* FROM i INNER JOIN w      ON w.status = 'active'     AND i.id = wish.id INNER JOIN r     ON i.id = r.id     AND r.member_id = 1 && r.status = 'active'  ORDER BY i.id DESC  LIMIT 0,50 

Please post your schema.

If you are using wish as an existence flag, try:

SELECT i.*, CASE WHEN w.id IS NOT NULL THEN 1 ELSE 0 END AS wish FROM i INNER JOIN r     ON i.id = r.id     AND r.member_id = 1 && r.status = 'active'  LEFT JOIN w      ON w.status = 'active'     AND i.id = w.id ORDER BY wish DESC  LIMIT 0,50 

You can use the same technique with a LEFT JOIN to a SELECT DISTINCT subquery. I assume you aren’t specifying the w.member_id because you want to know if any members have this? In this case, definitely use the SELECT DISTINCT. You should have an index with id as the first column on w as well in order for that to perform:

SELECT i.*, CASE WHEN w.id IS NOT NULL THEN 1 ELSE 0 END AS wish FROM i INNER JOIN r     ON i.id = r.id     AND r.member_id = 1 && r.status = 'active'  LEFT JOIN (     SELECT DISTINCT w.id     FROM w      WHERE w.status = 'active' ) AS w     ON i.id = w.id ORDER BY wish DESC  LIMIT 0,50