Question

UPDATE:

Just to mention it on a more visible place. When I changed IN for =, the query execution time went from 180 down to 0.00008 seconds. Ridiculous speed difference.

---

This SQL query takes 180 seconds to finish! How is that possible? is there a way to optimize it to be faster?

SELECT IdLawVersionValidFrom 
FROM question_law_version 
WHERE IdQuestionLawVersion IN 
  (
  SELECT MAX(IdQuestionLawVersion) 
  FROM question_law_version 
  WHERE IdQuestionLaw IN 
    (
    SELECT MIN(IdQuestionLaw) 
    FROM question_law 
    WHERE IdQuestion=236 AND IdQuestionLaw>63
    )
  )

There are only about 5000 rows in each table so it shouldn’t be so slow.

Answer 1

(Posting my comment as an answer as apparently it did make a difference!)

Any difference if you change the IN
to =?

If anyone wants to investigate this further I’ve just done a test and found it very easy to reproduce.

Create Table

CREATE TABLE `filler` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) 

Create Procedure

CREATE PROCEDURE `prc_filler`(cnt INT)
BEGIN
        DECLARE _cnt INT;
        SET _cnt = 1;
        WHILE _cnt <= cnt DO
                INSERT
                INTO    filler
                SELECT  _cnt;
                SET _cnt = _cnt + 1;
        END WHILE;
END

Populate Table

  call prc_filler(5000)

Query 1

SELECT id 
FROM filler 
WHERE id =  (SELECT MAX(id) FROM filler  WHERE id =   
 ( SELECT MIN(id) 
    FROM filler
    WHERE id between 2000 and 3000
    )
  )

Equals Explain Output http://img689.imageshack.us/img689/5592/equals.png

Query 2 (same problem)

SELECT id 
FROM filler 
WHERE id in  (SELECT MAX(id) FROM filler  WHERE id in   
 ( SELECT MIN(id) 
    FROM filler
    WHERE id between 2000 and 3000
    )
  )

In Explain Output http://img291.imageshack.us/img291/8129/52037513.png