Question

I know that I have to avoid to use “IN” clause, but I don’t know how to do in this case. The scenario is this: from PHP I get a list of values and then I construct a query like this:

SELECT id FROM
    ( SELECT * FROM `tips` r 
      LEFT JOIN (SELECT tip FROM `tips_showed` WHERE user='8') AS a ON r.id=a.tip
      WHERE a.tip IS NULL ) AS t
WHERE t.id IN
    ('3','4','5','2','6','7','8','9','10','18',
     '11','12','13','14','15','16','17','20') LIMIT 1

This makes my site very slow (because it is executed each time a user visits it). How can make it faster?

Answer 1

I believe you can simplify the query to:

SELECT t.id
    FROM `tips` t
        LEFT JOIN `tips_showed` a
            ON t.id = a.tip
                AND a.user = '8'
    WHERE a.tip IS NULL
        AND t.id IN ('3','4','5','2','6','7','8','9','10','18',
                     '11','12','13','14','15','16','17','20')
    LIMIT 1

Using NOT EXISTS instead of the LEFT JOIN may also buy you some additional performace. Try each and compare.

 SELECT t.id
     FROM `tips` t
     WHERE t.id IN ('3','4','5','2','6','7','8','9','10','18',
                    '11','12','13','14','15','16','17','20')
        AND NOT EXISTS(SELECT NULL 
                           FROM `tips_showed` a 
                           WHERE a.tip = t.id
                               AND a.user = '8')
    LIMIT 1