database paging design - sql

Question

I'm fetching data for my grid like this
SELECT
Orders.CustomerID,
Orders.OrderTime,
OrderItems.ProductID,
OrderItems.Quantity
FROM
dbo.Orders INNER JOIN dbo.OrderItems
ON Orders.ID = OrderItems.OrderID
I also need the total count for the pagination.
There're two options.
1- Do an another fetch
SELECT count(*) FROM dbo.Orders
2- Put the count statement in the query
SELECT
Orders.CustomerID,
Orders.OrderTime,
OrderItems.ProductID,
OrderItems.Quantity,
(SELECT count(*) FROM dbo.Orders) as Count
FROM
dbo.Orders INNER JOIN dbo.OrderItems
ON Orders.ID = OrderItems.OrderID
Which way should I go ?

Answer 1

Of the 2 methods you've put forward, the first (separate query) is better. The second method means the count will appear in every row returned which is a bit unnecessary. Also if the query returns 20 rows, the select count(*) will be executed 20 times (if i remember right, guess this could depend on which database engine you're using).
Additionally, depending on how much traffic you're envisaging and how big the table is likely to get, you can improve upon this by caching the result of select count(*) somewhere, and then refreshing it upon insertions / deletions to the table.

Answer 2

If this is for SQL Server 2005 or higher, one of the best ways to get pagination is to use a Common Table Expression.
CREATE PROC MyPaginatedDataProc
#pageNumber INT
AS
WITH OrdersCTE (CustomerID, OrderTime, ProductID, Quantity, RowNumber)
AS
(
SELECT
Orders.CustomerID,
Orders.OrderTime,
OrderItems.ProductID,
OrderItems.Quantity,
ROW_NUMBER() OVER (ORDER BY OrderItems.OrderID) AS RowNumber
FROM
dbo.Orders INNER JOIN dbo.OrderItems ON Orders.ID = OrderItems.OrderID
)
SELECT
CustomerID,
OrderTime,
ProductId,
Quantity
FROM
OrdersCTE
WHERE
RowNumber BETWEEN (#pageNumber * 10) AND (((#pageNumber + 1) * 10) -1)
Otherwise for getting the total row count, I'd use a separate query like Mailslut said.

Answer 3

If you are using oracle you can use COUNT(*) OVER ( ) CNT. This one was more efficient
as it takes single table scan
SELECT
Orders.CustomerID,
Orders.OrderTime,
OrderItems.ProductID,
OrderItems.Quantity,
COUNT(*) OVER ( ) CNT as Count
FROM
dbo.Orders INNER JOIN dbo.OrderItems
ON Orders.ID = OrderItems.OrderID

Answer 4

As #Mailslut suggests, you should probably use two queries. However, you should probably add a
WHERE clause to the query that fetches the data, so you only fetch the data that you actually need to show (unless you are caching it).
If more than one thread is accessing the database at a time, you will also need to somehow make sure that the count is kept in sync with the database.

Answer 5

I would consider something different, because what you are trying to do is not very simple, but quite necessary. Have you considered using the SQL Server row_number function? This way you will know how many records there are by looking at the max row_number returned, but also in the order you want.
SELECT
Orders.CustomerID,
Orders.OrderTime,
OrderItems.ProductID,
OrderItems.Quantity,
ROW_NUMBER() OVER(ORDER BY Orders.CustomerId) rn
FROM
dbo.Orders INNER JOIN dbo.OrderItems
ON Orders.ID = OrderItems.OrderID