Simple query very slow due to order by

Question

Please could anyone help with the following query? (180352 rows)

SELECT COUNT(p.stock_id) AS num_products,
p.master_photo, p.product_photo, p.stock_id, p.master,         


        

Answer 1

You haven't provided the output from explain, however based on your query it would seem that ORDER BY forces a full table scan. That would make the query very slow.

When you don't use the ORDER BY, the db reads the results for the first 20 master values (there maybe quite a few of them) and groups them together and returns the result.

When you order by stock_id the whole table needs to be looked at to find which masters are associated with the lowest values stock_ids

It maybe possible to improve performance with a composite index on sub_category,master however a conclusion cannot be made unless you share your SHOW CREATE TABLES, EXPLAIN output.

Update Based on your CREATE TABLE statements, I see that your database isn't normalized. For example Why do I get the feeling that the following columns should in a table of their own?

   supplier varchar(50) NOT NULL,
   supplier_stock_id varchar(50) NOT NULL,
   supplier_discount int(11) NOT NULL,

You should only have a supplier_stock_id in your products table (foreign key to the suppliers table). There are similar sets of columns which really should be moved out.

When you do so you can create leaner and meaner indexes on this table. But that's not all the table becomes narrower. Which in turn means the worst case scenario of a full table scan actually becomes a lot faster.

I also noticed that the table does not have a primary key. Which is a big no-no. The stock_id if it's numeric should be primary key. If it's not numeric it might stil be the best candidate for primary key but this is something you need to decide.

Answer 2

Try adding an Index on stock_id in the products table... that should help.