Perform subquery on another subquery's result in MySQL? - mysql

Question

Suppose there is table Product with columns prod_id, a, b
Suppose there is table Warehouse with columns w_id, pid, amount
The point is to join these two tables so that we would get in result columns prod_id, a, b, total (which is sum of the amount for pid = prod_id)
Currently I have this query:
SELECT
t1.prod_id, a, b, c AS total
FROM
(
SELECT prod_id, a, b
FROM product
WHERE ean = "3165142644363"
) t1
LEFT JOIN
(
SELECT warehouse.pid, SUM(amount) AS c
FROM warehouse
WHERE warehouse.pid IN (SELECT prod_id FROM product WHERE ean = "3165142644363")
GROUP BY warehouse.pid
) t2
ON t2.pid = t1.prod_id
Now you see there is two similar but not identical queries:
SELECT prod_id, a, b FROM product WHERE ean = "3165142644363"
SELECT prod_id FROM product WHERE ean = "3165142644363"
Executing both of these queries takes too much time. Is there a way to execute only the first one and then select product id from the result of that subquery? Or any other efficient way to rewrite the query. Creating the view is not an option as there can be many queries all with different ean parameter. The query returns more than one row of data for the same ean

Answer 1

I think you can remove the subquery appearing after WHERE ... IN:
SELECT
t1.prod_id, a, b, c AS total
FROM
(
SELECT prod_id, a, b
FROM product
WHERE ean = '3165142644363'
) t1
LEFT JOIN
(
SELECT warehouse.pid, SUM(amount) AS c
FROM warehouse
GROUP BY warehouse.pid
) t2
ON t2.pid = t1.prod_id
The reason you can remove it is that the t2 subquery would only contain pid values which appear in the t1 subquery. Otherwise, the sum c would just be zero anyway after the left join. A left join already implicitly handles the case where records from product may not join to anything in the t2 subquery.
That being said, removing the WHERE ... IN subquery might not improve performance that much, because it is not correlated anyway. Most likely the optimizer would only need to compute it once and cache it somewhere.

Answer 2

Eventually found a solution myself. Way easier than I thought:
SELECT pid, a, b, SUM(amount) AS total
FROM product
LEFT JOIN warehouse USING(pid)
WHERE ean = "3165142644363"
GROUP BY pid
In my example there are columns prod_id and pid, but in fact it is pid in both tables

Answer 3

Here's another formulation that might be better:
SELECT pid, a, b,
( SELECT SUM(amount)
FROM wherehouse
WHERE pid = product.prod_id
) AS total
FROM product
WHERE ean = "3165142644363";
With indexes:
product: INDEX(ean)
wherehouse: INDEX(pid, amount) -- in this order