Optimize query with multiple subquery joins

Question

I have a query that gets the product description from product table (1st select) then subtracts it to the following subquery statements to get the no. of stocks remaining:

-sum of each product bought in the inventory table (2nd)

-sum of each product sold in the sales_detail table (3rd)

-sum of each product transferred to another branch in the stock_transfer table (4th)

-sum of each product that got damaged in the damaged_product table (5th)

The problem is every time this query loads, it goes full search of all 4 tables to get the sum of the quantity columns. And as time goes by, more records are stored and the query will become slower. Any suggestions?

SELECT p.Id,p.Product_Name Product,p.Description, c.Category_Name Category,sc.Subcategory_Name Subcategory,s.Supplier_Name Supplier, p.Selling_Price `Unit Price`,i.Stocks,s.Sales, i.Stocks - IFNULL(s.Sales, 0) - IFNULL(t.Transfer, 0) - IFNULL(d.Damage, 0) AS Remaining

FROM (SELECT Id, Product_Name, Description, Selling_Price, Category_Id, Subcategory_Id, Supplier_Id       FROM product WHERE enable_flag = 1) p

LEFT OUTER JOIN(SELECT product_id, COALESCE(SUM(quantity), 0) AS Stocks
                FROM inventory
                WHERE enable_flag = 1 GROUP BY product_id) i ON p.Id = i.product_id

LEFT OUTER JOIN(SELECT product_id, COALESCE(SUM(quantity), 0) AS Sales
                FROM sales_detail
                WHERE enable_flag = 1 GROUP BY product_id) s USING(product_id)

LEFT OUTER JOIN(SELECT product_id, COALESCE(SUM(transfer_quantity), 0) AS Transfer
                FROM stock_transfer
                WHERE enable_flag = 1 GROUP BY product_id) t USING(product_id)

LEFT OUTER JOIN(SELECT product_id, COALESCE(SUM(damaged_quantity), 0) AS Damage
                FROM damaged_product
                WHERE enable_flag = 1 GROUP BY product_id) d USING(product_id)

JOIN Category c ON p.Category_Id=c.Id

JOIN Subcategory sc ON p.Subcategory_Id=sc.Id

JOIN Supplier s ON p.Supplier_Id=s.Id;

Answer 1

The use of subqueries prevents the use of indexes, which can slow down a query. I would suggest a query such as this:

SELECT p.*, sc.Subcategory_Name as Subcategory, s.Supplier_Name as Supplier,
       p.Selling_Price as `Unit Price`,
       (SELECT COALESCE(SUM(quantity), 0) 
        FROM inventory i
        WHERE s.enable_flag = 1 AND s.product_id = p.product_id
       ) as stocks,
       (SELECT COALESCE(SUM(quantity), 0) 
        FROM sales_detail sd
        WHERE i.enable_flag = 1 AND i.product_id = p.product_id
       ) as sales,
       (SELECT COALESCE(SUM(tansferquantity), 0) 
        FROM stock_transfer st
        WHERE st.enable_flag = 1 AND st.product_id = p.product_id
       ) as transfers,
       (SELECT COALESCE(SUM(damaged_quantity), 0) 
        FROM damage d
        WHERE d.enable_flag = 1 AND d.product_id = p.product_id
       ) as damaged
FROM product p JOIN
     Category c
     ON p.Category_Id = c.Id JOIN
     Subcategory sc
     ON p.Subcategory_Id = sc.Id JOIN
     Supplier s
     ON p.Supplier_Id = s.Id
WHERE p.enable_flag = 1;

For performance, the underlying tables want indexes on (product_id, enable_flag, quantity) . MySQL can use the index for aggregation in a correlated subquery.

I realize that you also have calculated values. You may need to do these calculations in an outer query, taking the hit of an additional materialization of the subquery.