Top N Per Group with Multiple Table Joins

Question

Based on my research, this is a very common problem which generally has a fairly simple solution. My task is to alter several queries from get all results into

Answer 1

Even though you specify LIMIT 100, this type of query will require a full scan and table to be built up, then every record inspected and row numbered before finally filtering for the 100 that you want to display.

select
    vendorid, productid, NumSales
from
(
    select
        vendorid, productid, NumSales,
        @r := IF(@g=vendorid,@r+1,1) RowNum,
        @g := vendorid
    from (select @g:=null) initvars
    CROSS JOIN 
    (
        SELECT COUNT(oi.price) AS NumSales, 
               p.productid, 
               p.vendorid
        FROM products p
        INNER JOIN vendors v ON (p.vendorid = v.vendorid)
        INNER JOIN orders_items oi ON (p.productid = oi.productid)
        INNER JOIN orders o ON (oi.orderid = o.orderid)
        WHERE (p.Approved = 1 AND p.Active = 1 AND p.Deleted = 0)
        AND (v.Approved = 1 AND v.Active = 1 AND v.Deleted = 0)
        AND o.`Status` = 'SETTLED'
        AND o.Deleted = 0
        GROUP BY p.vendorid, p.productid
        ORDER BY p.vendorid, NumSales DESC
    ) T
) U
WHERE RowNum <= 3
ORDER BY NumSales DESC
LIMIT 100;

The approach here is

1. Group by to get NumSales
2. Use variables to row number the sales per vendor/product
3. Filter the numbered dataset to allow for a max of 3 per vendor
4. Order the remaining by NumSales DESC and return only 100