paging over SELECT UNION super slow and killing my server - sql-server

Question

I have an SP that returns paged data from a query that contains a UNION. This is killing my DB and taking 30 seconds to run sometimes, am I missing something obvious here? What can I do to improve it's performance?
Tables Involved: Products, Categories, CategoryProducts
Goal:
Any Products that are not in a Category or have been deleted from a category UNION all Products currently in a category and page over them for a web service.
I have Indexes on all columns that I am joining on and there are 427,996 Products, 6148 Categories and 409,691 CategoryProducts in the database.
Here is my query that is taking between 6, and 30 seconds to run:
SELECT * FROM (
SELECT ROW_NUMBER() OVER(ORDER BY Products.ItemID, Products.ManufacturerID) AS RowNum, *
FROM
(
SELECT Products.*,
CategoryID = NULL, CategoryName = NULL,
CategoryProductID = NULL,
ContainerMinimumQuantity =
CASE COALESCE(Products.ContainerMinQty, 0)
WHEN 0 THEN Products.OrderMinimumQuantity
ELSE Products.ContainerMinQty
END
Products.IsDeleted,
SortOrder = NULL
FROM CategoryProducts RIGHT OUTER JOIN Products
ON CategoryProducts.ManufacturerID = Products.ManufacturerID
AND CategoryProducts.ItemID = Products.ItemID
WHERE (Products.ManufacturerID = #ManufacturerID)
AND (Products.ModifiedOn > #tStamp )
AND ((CategoryProducts.IsDeleted = 1) OR (CategoryProducts.IsDeleted IS NULL))
UNION
SELECT Products.*,
CategoryProducts.CategoryID , CategoryProducts.CategoryName,
CategoryProducts.CategoryProductID ,
ContainerMinimumQuantity =
CASE COALESCE(Products.ContainerMinQty, 0)
WHEN 0 THEN Products.OrderMinimumQuantity
ELSE Products.ContainerMinQty
END
CategoryProducts.IsDeleted,
CategoryProducts.SortOrder
FROM Categories INNER JOIN
CategoryProducts ON Categories.CategoryID = CategoryProducts.CategoryID INNER JOIN
Products ON CategoryProducts.ManufacturerID = Products.ManufacturerID
AND CategoryProducts.ItemID = Products.ItemID
WHERE (Products.ManufacturerID = #ManufacturerID)
AND (Products.ModifiedOn > #tStamp OR CategoryProducts.ModifiedOn > #tStamp))
AS Products) AS C
WHERE RowNum >= #StartRow AND RowNum <= #EndRow
Any insight would be greatly appreciated.

Answer 1

If I read your situation correctly, the only reason for having two distinct queries is treatment of missing/deleted CategoryProducts. I tried to address this issue by left join with IsDeleted = 0 to bring all deleted CategoryProducts to nulls, so I don't have to test them again. ModifiedOn part got another test for null for missing/deleted Categoryproducts you wish to retrieve.
select *
from (
SELECT
Products.*,
-- Following three columns will be null for deleted/missing categories
CategoryProducts.CategoryID,
CategoryProducts.CategoryName,
CategoryProducts.CategoryProductID ,
ContainerMinimumQuantity = COALESCE(nullif(Products.ContainerMinQty, 0),
Products.OrderMinimumQuantity),
CategoryProducts.IsDeleted,
CategoryProducts.SortOrder,
ROW_NUMBER() OVER(ORDER BY Products.ItemID,
Products.ManufacturerID) AS RowNum
FROM Products
LEFT JOIN CategoryProducts
ON CategoryProducts.ManufacturerID = Products.ManufacturerID
AND CategoryProducts.ItemID = Products.ItemID
-- Filter IsDeleted in join so we get nulls for deleted categories
-- And treat them the same as missing ones
AND CategoryProducts.IsDeleted = 0
LEFT JOIN Categories
ON Categories.CategoryID = CategoryProducts.CategoryID
WHERE Products.ManufacturerID = #ManufacturerID
AND (Products.ModifiedOn > #tStamp
-- Deleted/missing categories
OR CategoryProducts.ModifiedOn is null
OR CategoryProducts.ModifiedOn > #tStamp)
) C
WHERE RowNum >= #StartRow AND RowNum <= #EndRow
On a third look I don't see that Category is used at all except as a filter to CategoryProducts. If this is the case second LEFT JOIN should be changed to INNER JOIN and this section should be enclosed in parenthessis.