Increasing the speed of a query with a sub select? - mysql

Question

I'm have a question. The following query is taking upwards of 2 - 3 seconds to exicute and I'm not sure why. I have 2 tables involved one with a list of items and the another with a list of attribute's for each item. The items table is indexed with unique primary key and the attributes table has a foreign key constraint.
The relationship between the items table is ONE TO MANY to the attributes.
I am not sure how else to speed up query and would appreciate any advice.
The database is MYSQL inodb
EXPLAIN SELECT * FROM eshop_items AS ite WHERE (SELECT attValue FROM eshop_items_attributes WHERE attItemId=ite.ItemId ANd attType=5 AND attValue='20')='20' ORDER BY itemAdded DESC LIMIT 0, 18;
id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra
1 PRIMARY ite ALL NULL NULL NULL NULL 57179 Using where; Using filesort
2 DEPENDENT SUBQUERY eshop_items_attributes ref attItemId attItemId 9 gabriel_new.ite.itemId 5 Using where
Index: eshop_items_attributes
Name Fieldnames Index Type Index method
attItemId attItemId Normal BTREE
attType attType Normal BTREE
attValue attValue Normal BTREE
Index: eshop_items
Name Fieldnames Index Type Index method
itemCode itemCode Unique BTREE
itemCodeOrig itemCodeOrig Unique BTREE
itemConfig itemConfig Normal BTREE
itemStatus itemStatus Normal BTREE
Can't use a join because the item_attributes table is a key -> value pair table. So for every record in the items_attributes table there can be many item id's
here is a sample
item_id attribute_index attribute_value
12345 10 true
12345 2 somevalue
12345 6 some other value
32456 10 true
32456 11 another value
32456 2 somevalue
So a join wouldn't work because I can't join multiple rows from the items_attributes table to one row in the items table.
I can't write a query where attribute_index is = to 2 AN attribute_index = 10. I would always get back no results.
:(

Answer 1

Change the query from correlated to IN and see what happens.
SELECT *
FROM eshop_items AS ite
WHERE ItemId IN (
SELECT attItemId
FROM eshop_items_attributes
WHERE attType=5
AND attValue='20')
ORDER BY itemAdded DESC
LIMIT 0, 18
You'll see further gains by changing your btree to bitmap on eshop_items_attributes. But be warned: bitmap has consequences on INSERT/UPDATE.

Answer 2

The "DEPENDENT SUBQUERY" is what's killing performance in this query. It has to run the subquery once for every distinct ItemId in the outer query. It should be much better as a join:
SELECT ite.* FROM eshop_items AS ite
INNER JOIN eshop_items_attributes AS a ON ite.ItemId = a.attItemId
WHERE a.attType = 5 AND a.attValue = 20
ORDER BY ite.itemAdded DESC LIMIT 0, 18;

Answer 3

I find it much easier to think about such a query as a join:
SELECT ite.*
FROM eshop_items ite join
eshop_items_attributes ia
on ia.attItemId = ite.ItemId and
ia.attType = 5 and
ia.attValue='20'
ORDER BY ite.itemAdded DESC
LIMIT 0, 18;
This works if there is at most one matching attribute for each item. Otherwise, you need select distinct (which could hurt performance, except you are already doing a sort).
To facilitate this join, create the index eshop_items_attributes(attType, attValue, attItemId). The index should satisfy the join without having to read the table, the rest is dealing with the result set.
The same index would probably help with the correlated subquery.