Optimizing a large MySQL query

Question

Today I was told by my hosting provider that my database must be blocked immediately. I have a query that causes huge load on the server. Do you know how can I optimize my query just to make the server less loaded?

SELECT `users`.`id`,
       `why_me`, 
       `created`,
       `thumbnail`,
       `rating_date`,
       CONCAT(first_name, ' ', last_name) AS name,
       SUBSTRING(why_me, 1, 27) as subwhy,
       COUNT(`rating_date`) AS total,
       MAX(`rating_date`) AS maxrate
  FROM (`user_profiles`)
  LEFT OUTER JOIN `rates` ON `user_profiles`.`id`=`rates`.`user_id`
  JOIN `users` ON `user_profiles`.`user_id`=`users`.`id`
  WHERE `users`.`activated` =  '1'
      AND `last_name` != ""
      AND `first_name` != ""
      AND  concat_ws(' ', first_name, last_name) COLLATE UTF8_GENERAL_CI  LIKE '%%'
  GROUP BY `user_profiles`.`user_id`
  ORDER BY `total` desc

I'd appreciate any help from you. Thanks!

Answer 1

AND  concat_ws(' ', first_name, last_name) COLLATE UTF8_GENERAL_CI  LIKE '%%'

has no effect but is expensive (long running). It scans all the rows one-by-one without returning anything usefull. Remove!

Log in into phpmyadmin and run this query with " EXPLAIN " prepended, ie "EXPLAIN SELECT users...". Post the result. It will show us if SQL can directly adress the required rows, or if it searching them one-by-one. Look out for the row used keys: if it´s empty, you need additioanl indeces:

1) Make sure you have an index on every column used for the joins, ie rates.user_idand user_profiles.user_id.

2) Make sure you have indeces on everything used for the where part with a high cardinality , ie first_name and last_name.

3) Even better: Create an index over 3 columns : activated,first_name,last_name

4) The GROUP and ORDER BY are causing the buildt of a temporary table. Can you port some of this logic into PHP ?

5) Cache the result. That´s nothing to be done in real time!