Question

I have an SQL query that I think is not optimized enough. I timed it, and it sometimes take 1.5 seconds to complete, which seems a little high, no?

anyway this is the query:

SELECT id, link, feed, category, description, title, GROUP_CONCAT( tag ) as t, published
            FROM items
            LEFT JOIN tags ON items.id = tags.item_id
            WHERE id NOT IN (
                SELECT item_id
                FROM tags, sinunim
                WHERE tag = name
                AND op = '1'
                AND user = '$user_cookie'
            ) AND id NOT IN (
                SELECT id
                FROM sinunim
                WHERE id <> 0
                AND user = '$user_cookie'
            ) AND id NOT IN (
                SELECT i.id
                FROM  sinunim s, items i
                WHERE s.type =  'category'
                AND s.name = i.category
                AND s.op = '1'
                AND s.user = '$user_cookie'
            ) AND id NOT IN (
                SELECT i.id
                FROM  `sinunim` s, items i
                WHERE s.name = i.feed
                AND s.op = '1'
                AND s.user = '$user_cookie'
            )
            GROUP BY items.title
            ORDER BY items.published DESC
            LIMIT 0 , 50

Answer 1

You can rewrite the 4 NOT IN sub queries as one single sub query and since you’re grouping by items.title you can even eliminate all sub queries and use joins instead.

That way whatever you join will be joined only once and you can recreate the various comparisons by grouping them in one bigger logical expression (x = y AND y = z) OR (k = l AND m = n).