Rows as columns with lookup table - mysql

Question

I have two tables that look like this:
Table cars
+--------+-----------+-------+---------+
| car_id | attribute | value | brand |
+--------+-----------+-------+---------+
| 1 | colore | rosso | Ferrari |
| 1 | prezzo | 100 | Ferrari |
| 2 | couleur | bleu | Renault |
| 2 | prix | 50 | Renault |
| 3 | colore | blu | Ferrari |
| 3 | prezzo | 100 | Ferrari |
+--------+-----------+-------+---------+
Table translations
+--------------------+----------------+---------+----------------------+------------------+
| original_attribute | original_value | brand | translated_attribute | translated_value |
+--------------------+----------------+---------+----------------------+------------------+
| colore | rosso | Ferrari | color | red |
| prezzo | 100 | Ferrari | price | 100 |
| colore | blu | Ferrari | color | blue |
| couleur | bleu | Renault | color | blue |
| prix | 50 | Renault | price | 50 |
+--------------------+----------------+---------+----------------------+------------------+
I am trying to get to a table that looks like this:
+-------------------+-------+-------------+--------------------+
| translated_car_id | color | price | translated_brand |
+-------------------+-------+-------------+--------------------+
| 1 | red | 100 | Ferrari |
| 2 | blue | 50 | Renault |
| 3 | blue | 100 | Ferrari |
+-------------------+-------+-------------+--------------------+
At the moment, I am using below code. It works, but is excruciatingly slow.
SELECT
car_id translated_car_id,
MAX(CASE
WHEN translations.translated_attribute = 'color' THEN translations.translated_value
END) color,
MAX(CASE
WHEN translations.translated_attribute = 'price' THEN translations.translated_value
END) price,
brand translated_brand
FROM
cars c
INNER JOIN
translations ON (c.attribute = translations.original_attribute
AND c.brand = translations.brand
AND c.value = relations.original_value)
GROUP BY c.car_id
Anybody have an idea on how to make the query or the structure more efficient? Would really appreciate it.
Thanks in advance

Answer 1

From what I can find MySql doesn't have hash matches, which would be useful here. So I'm assuming that everything is being done as nested loop joins.
My concern is that because there isn't an index on translations, for every row in cars it has to scan the translations table to find the matching rows.
I would recommend a clustered index on translations(original_value, brand, original_attribute). (ms-sql does best with most specific first everything else being equal, not sure about MySql) That way it can go right to the row that it needs to match to. That should allow queries with one car to be done quickly.
If you can reduce the translations from manufacturer to language that would certainly help with the size of the translation table, but you would have to be sure that it works with your dataset because it does take away a level of flexibility.
I think that MySql will be able to use the index on cars to process the GROUP BY efficiently, but I will propose a more normalized schema where you don't need the group by.
car
-------------
car_id
brand
car_attribute
------------
car_id
attribute
value
clustered_index(car_id, attribute)
brand_attribute
------------
brand_attribute_id
brand
attribute
translated_attribute
clustered_index(brand, translated_attribute)
brand_attribute_value
------------
brand_attribute_id
value
translated_value
clustered_index(brand_attribute_id, value)
And here's the query to do it that way.
SELECT
car.car_id,
color_brand_value.translated_value AS color,
price_brand_value.translated_value AS price,
car.brand
FROM
car
INNER JOIN brand_attribute AS color_brand
ON color_brand.brand = car.brand
AND color_brand.translated_attribute = 'color'
INNER JOIN car_attribute AS color_attribute
ON color_attribute.car_id = car.car_id
AND color_attribute.attribute = brand.attribute
INNER JOIN brand_attribute_value AS color_brand_value
ON color_brand_value.brand_attribute_id = color_brand.brand_attribute_id
AND color_brand_value.value = color_attribute.value
INNER JOIN brand_attribute AS price_brand
ON price_brand.brand = car.brand
AND price_brand.translated_attribute = 'price'
INNER JOIN car_attribute AS price_attribute
ON price_attribute.car_id = car.car_id
AND price_attribute.attribute = brand.attribute
INNER JOIN brand_attribute_value AS price_brand_value
ON price_brand_value.brand_attribute_id = price_brand.brand_attribute_id
AND price_brand_value.value = price_attribute.value
It's definitely more complicated to do it this way. With your situation I'm not sure it's better, but something to consider if the first option isn't good enough.
My background is in mssql, so there may be differences that I'm not aware of. Please leave comments if I've missed or got something wrong.