PostgreSQL function/query infinite execution

So I tried to use a function I created into a query and when I execute the query it runs forever. The function finds the customer that his orders had the biggest quantity of products in it. The query returns him along with any other customer that has ordered the same amount of products.

Here is the function:

CREATE OR REPLACE FUNCTION max_orderQty ()
RETURNS bigint
AS $$
     SELECT SUM(OrderQty)
     FROM Customer,SalesOrderHeader,SalesOrderDetail
     WHERE Customer.CustomerID = SalesOrderHeader.CustomerID
     AND SalesOrderHeader.SalesOrderID = SalesOrderDetail.SalesOrderID
     GROUP BY SalesOrderHeader.CustomerID
     ORDER BY SUM(OrderQty) DESC LIMIT 1;
$$ LANGUAGE SQL;

Here is the the query:

SELECT SUM(OrderQty),SalesOrderHeader.CustomerID
FROM Customer,SalesOrderHeader,SalesOrderDetail
WHERE Customer.CustomerID = SalesOrderHeader.CustomerID
AND SalesOrderHeader.SalesOrderID = SalesOrderDetail.SalesOrderID
GROUP BY SalesOrderHeader.CustomerID
HAVING SUM(OrderQty) = max_orderQty();

Also this works and it solves the same thing:(in 220ms)

SELECT SUM(OrderQty),SalesOrderHeader.CustomerID
FROM Customer,SalesOrderHeader,SalesOrderDetail
WHERE Customer.CustomerID = SalesOrderHeader.CustomerID
AND SalesOrderHeader.SalesOrderID = SalesOrderDetail.SalesOrderID
GROUP BY SalesOrderHeader.CustomerID
HAVING SUM(OrderQty) = (SELECT SUM(OrderQty)
                        FROM Customer,SalesOrderHeader,SalesOrderDetail
                        WHERE Customer.CustomerID = SalesOrderHeader.CustomerID
                        AND SalesOrderHeader.SalesOrderID = SalesOrderDetail.SalesOrderID
                        GROUP BY SalesOrderHeader.CustomerID
                        ORDER BY SUM(OrderQty) DESC LIMIT 1);

So when I tried to split it to function/query it just runs forever and I cannot find the reason why.And I thought using a function instead would be faster.Is that wrong?

Any help is very appreciated. Thanks in advance.

Solution

There is no reason to think that a function would be faster. Your problem is that the function is being called for each row in the output -- and the calculation is being re-done each time.

You should be able to fix this by declaring the function to be STABLE; the default is VOLATILE.

You can read about these nuances in the documentation.

EDIT:

You can force it to run faster by only calling the function once:

SELECT SUM(OrderQty),SalesOrderHeader.CustomerID
FROM Customer JOIN
     SalesOrderHeader
     ON Customer.CustomerID = SalesOrderHeader.CustomerID JOIN
     SalesOrderDetail
     ON SalesOrderHeader.SalesOrderID = SalesOrderDetail.SalesOrderID CROSS JOIN
     (SELECT max_orderQty() as maxoq) x
GROUP BY SalesOrderHeader.CustomerID
HAVING SUM(OrderQty) = MAX(maxoq);