Question

What more can I do to optimize this query?

SELECT * FROM     (SELECT `item`.itemID, COUNT(`votes`.itemID)  AS `votes`,            `item`.title, `item`.itemTypeID, `item`.            submitDate, `item`.deleted, `item`.ItemCat,            `item`.counter, `item`.userID, `users`.name,            TIMESTAMPDIFF(minute,`submitDate`,NOW()) AS 'timeMin' ,            `myItems`.userID as userIDFav, `myItems`.deleted as myDeleted       FROM    (votes `votes` RIGHT OUTER JOIN item `item`                   ON (`votes`.itemID = `item`.itemID))            INNER JOIN               users `users`            ON (`users`.userID = `item`.userID)     LEFT OUTER JOIN               myItems `myItems`            ON (`myItems`.itemID = `item`.itemID)      WHERE (`item`.deleted = 0)      GROUP BY `item`.itemID,               `votes`.itemID,               `item`.title,               `item`.itemTypeID,               `item`.submitDate,               `item`.deleted,               `item`.ItemCat,               `item`.counter,               `item`.userID,               `users`.name,               `myItems`.deleted,               `myItems`.userID     ORDER BY `item`.itemID DESC) as myTable where myTable.userIDFav = 3 or myTable.userIDFav is null             limit 0, 20  

I’m using MySQL

Thanks

Answer 1

Of course, as @theomega said, look at the execution plan.

But I’d also suggest to try and ‘clean up’ your statement. (I don’t know which one is faster – that depends on your table sizes.) Usually, I’d try to start with a clean statement and start optimizing from there. But typically, a clean statement makes it easier for the optimizer to come up with a good execution plan.

So here are some observations about your statement that might make things slow:

• a couple of outer joins (makes it hard for the optimzer to figure out an index to use)
• a group by
• a lot of columns to group by

As far as I understand your SQL, this statement should do most of what yours is doing:

SELECT `item`.itemID, `item`.title, `item`.itemTypeID, `item`.        submitDate, `item`.deleted, `item`.ItemCat,        `item`.counter, `item`.userID, `users`.name,        TIMESTAMPDIFF(minute,`submitDate`,NOW()) AS 'timeMin'    FROM    (item `item` INNER JOIN users `users`        ON (`users`.userID = `item`.userID) 

WHERE

Of course, this misses the info from the tables you outer joined, I’d suggest to try to add the required columns via a subselect:

SELECT `item`.itemID,         (SELECT count (itemID)         FROM votes v        WHERE v.itemID = 'item'.itemID) as 'votes', <etc.> 

This way, you can get rid of one outer join and the group by. The outer join is replaced by the subselect, so there is a trade-off which may be bad for the ‘cleaner’ statement.

Depending on the cardinality between item and myItems, you can do the same or you’d have to stick with the outer join (but no need to reintroduce the group by).

Hope this helps.