Question

I have this query which takes 27 seconds to execute:

SELECT ocal_files.*, count(DISTINCT ocal_favs.username) as favs 
FROM ocal_files
   INNER JOIN ocal_favs on ocal_favs.clipart_id = ocal_files.id 
GROUP BY ocal_files.id 
ORDER BY favs DESC​

(instead of username it should be user_id, because I have table for users)

ocal_files has 37457 rows and ocal_favs has 18263

EDIT result of explain

mysql> EXPLAIN SELECT ocal_files.*, count(DISTINCT ocal_favs.username) as favs FROM ocal_files INNER JOIN ocal_favs on ocal_favs.clipart_i
d = ocal_files.id GROUP BY ocal_files.id ORDER BY favs DESC;                                                                             
+----+-------------+------------+--------+----------------+---------+---------+---------------------------------+-------+---------------------------------+
| id | select_type | table      | type   | possible_keys  | key     | key_len | ref                             | rows  | Extra                           |
+----+-------------+------------+--------+----------------+---------+---------+---------------------------------+-------+---------------------------------+
|  1 | SIMPLE      | ocal_favs  | ALL    | rlb_clipart_id | NULL    | NULL    | NULL                             | 18622 | Using temporary; Using filesort|
|  1 | SIMPLE      | ocal_files | eq_ref | PRIMARY        | PRIMARY | 4       | openclipart.ocal_favs.clipart_id |     1 | Using where                    |
+----+-------------+------------+--------+----------------+---------+---------+---------------------------------+-------+---------------------------------+
2 rows in set (0.00 sec)

Why is it slow? Can it be optimized? If yes then how?

Answer 1

Try to create an index on

ocal_favs ( clipart_id, username )

and make sure that there is a NOT NULL constraint on ocal_favs.username or add ocal_favs.username IS NOT NULL as a condition.

This should allow to get all information from ocal_files and that index.