SQL Server 2005 Paging Performance Tip

This quick tip demonstrates how to get the total rows as part of the paging query as well as how avoid a common coding error with joins that can harm performance. It also provides a faster alternative to the SQL Server's paging syntax using TABLE variables.

I've seen the following technique in several beginner code samples for demonstrating SQL Server 2005's ability to return paged results.

I've added the TotalRows = Count(*) OVER() line to demonstrate how return the total rows returned above and beyond the row count for the paged set.  This removes the need for a second query to get the total rows available for paging techniques in your application.  In your application, just check to make sure your resultset has records, then just grab the first record and retrieve its TotalRows column value.

Notice that in this query, the JOIN between the Orders table and the Users table is being run across all records that are found NOT just the records returned in the paged set.

  declare @StartRow int
  declare @MaxRows int

  select @StartRow = 1
  select @MaxRows = 10

   select *
     from
     (select o.*,u.FirstName,u.LastName,
         TotalRows=Count(*) OVER(),
         ROW_NUMBER() OVER(ORDER BY o.CreateDateTime desc) as RowNum
         from Orders o , Users u
   WHERE o.CreateDateTime > getdate() -30
        AND (o.UserID = u.UserID)
     )
     WHERE RowNum BETWEEN @StartRow AND (@StartRow + @MaxRows) -1

If you adjust your query as follows, you will see a substantial boost in performance.  Notice this query only performs the join on the returned resultset which is much, much smaller.

SELECT MyTable.*,u.FirstName,u.LastName
FROM
   (SELECT o.*,
       TotalRows=Count(*) OVER(),
       ROW_NUMBER() OVER(ORDER BY o.CreateDateTime desc) as RowNum
    FROM Orders o
    WHERE o.CreateDateTime > getdate() -30
   ) as MyTable, Users u
WHERE RowNum BETWEEN @StartRow AND (@StartRow + @MaxRows) -1
  and (MyTable.UserID = u.UserID)

Having recently reviewed ways to speed up the paging grids at EggHeadCafe (they were never slow but I'm always looking for ways to optimize things), I decided to put SQL Server's new paging mechanism to a test between itself and using standard queries in conjunction with TABLE variables.  Across 5 different very large tables at EggHeadCafe,  the TABLE variable option performed twice as fast as the suggested paging mechanism above.  Why I didn't of think of why much sooner is beyond me...

When you look at the inner query in the paging sample above, you notice that it is pulling back the entire Orders record for every single record in the Orders table.  Then, the outer query queries the inner table's results.

The
TABLE variable sample stored procedure syntax doesn't perform the same JOINS as above.  It is included
here just to give you an idea of how you might write your own.  Of course, depending on how often your data changes, you could even implement a cache of the primary keys to speed this up even more.

I saw the biggest performance gains when querying for pages deep in the result set.  In our case, that was tens of thousands of pages deep.

CREATE PROCEDURE [dbo].[GetRecordsPaged]
(
  @StartRow int,
  @MaxRows int
)
AS

declare @TotalRows bigint

declare @Pager table
(
  RowNumber int IDENTITY (1, 1) Primary key NOT NULL ,
  RecordID bigint,

Primary Key Clustered(RowNumber)
)

-- Notice that this INSERT INTO query can get 100% of its results from the clustered primary key index.

INSERT INTO @Pager (RecordID)
SELECT RecordID
FROM dbo.Record
ORDER BY RecordID desc --
You can ORDER BY datetime columns if your primary key is not a number oriented column.

SELECT top 1 @TotalRows = COUNT(*) from @Pager -- Did this because it is a little faster than TotalRows=Count(*) OVER()

-- You would append your JOINS etc. to the result query below

SELECT Record.*,
              @TotalRows as TotalRows
FROM dbo.Record
WHERE RecordID in (SELECT RecordID
                                     FROM @Pager i
                                   WHERE i.RowNumber BETWEEN @StartRow AND (@StartRow + @MaxRows) - 1)
ORDER BY RecordID desc

By Robbe Morris   Popularity  (7016 Views)
Picture
Biography - Robbe Morris
Robbe has been a Microsoft MVP in C# since 2004. He is also the co-founder of NullSkull.com which provides .NET articles, book reviews, software reviews, and software download and purchase advice.  Robbe also loves to scuba dive and go deep sea fishing in the Florida Keys or off the coast of Daytona Beach. Microsoft MVP