Question

i’m trying to get optimize an very old query that i can’t wrap my head around. the result that i want to archive is that i want to recommend the visitor on a web shop what other customers have shown interest in, i.e. what else they have bought together with the product that the visitor is looking at.

i have a subquery but it’s very slow, takes ~15s on ~8 000 000 rows.

the layout is that all products that are put in a users basket are kept in a table wsBasket and separated by a basketid (which in another table is associated with a member).

in this example i want to list all the most popular products that users have bought together with productid 427, but not list the productid 427 itself.

SELECT productid, SUM(quantity) AS qty 
FROM wsBasket 
WHERE basketid IN 
    (SELECT basketid 
     FROM wsBasket 
     WHERE productid=427) AND productid!=427 
GROUP by productid 
ORDER BY qty 
DESC LIMIT 0,4;

any help is much appreciated! hope this makes any sense at all to at least someone 🙂

 

UPDATE 1:
thanks for your comments guys here are my answers, they didn’t fit in the comments-field.

Using EXPLAIN on the above query i got the fllowing. Please note, I do not have any indexes on the table (except for primary key on the id-field), i want to modify the query to benefit from indexes and place indexes on the right keys.

+----+--------------------+----------+------+---------------+------+---------+------+------+----------------------------------------------+
| id | select_type        | table    | type | possible_keys | key  | key_len | ref  | rows | Extra                                        |
+----+--------------------+----------+------+---------------+------+---------+------+------+----------------------------------------------+
|  1 | PRIMARY            | wsBasket | ALL  | NULL          | NULL | NULL    | NULL | 2821 | Using where; Using temporary; Using filesort |
|  2 | DEPENDENT SUBQUERY | wsBasket | ALL  | NULL          | NULL | NULL    | NULL | 2821 | Using where                                  |
+----+--------------------+----------+------+---------------+------+---------+------+------+----------------------------------------------+

Answer 1

Two obvious indexes to add: one on basketid and a second on productid: then retry the query and a new EXPLAIN to see that the indexes are being used