Question

I’m using SQL Server 2008 and the following query to implement paged data retrieval from our JSF application, in below code i am retrieving 25 rows at a time sorted by the default sort column in DESC order.

SELECT * FROM 
    (  
        SELECT TOP 25 * FROM 
        ( 
            SELECT TOP 25  ...... WHERE CONDITIONS 
            --ORDER BY ... DESC
        )AS INNERQUERY  ORDER BY INNERQUERY.... ASC
    ) 
AS OUTERQUERY 
ORDER BY OUTERQUERY.... DESC 

It works, but with one obvious flow. If the users request to see the last page and there are over 10 million records in table, then the second TOP Query will have to first retrieve the 10 million records and only then the first top Query will pick out the Top 25 which will look like:

SELECT * FROM 
    (  
        SELECT TOP 25 * FROM 
        ( 
            SELECT TOP 10000000  ...... WHERE CONDITIONS 
            --ORDER BY ... DESC
        )AS INNERQUERY  ORDER BY INNERQUERY.... ASC
    ) 
AS OUTERQUERY 
ORDER BY OUTERQUERY.... DESC 

I looked into replacing the above with ROW_NUMBER OVER(….) but seemingly i had the same issue where the second TOP statement will have to get the entire result and only then you can do a where ROW_NUMBER between x and y.

Can you please point out my mistakes in the above approach and hints on how it can be optimized?

Answer 1

I’m currently using the following to code to retrieve subset of rows:

WITH PAGED_QRY (
                   SELECT *, ROW_NUMVER() OVER(ORDER BY Y) AS ROW_NO
                   FROM TABLE WHERE ....
               ) 
SELECT * FROM PAGED_QRY WHERE ROW_NO BETWEEN @CURRENT_INDEX and @ ROWS_TO_RETRIEVE
ORDER BY ROW_NO

where @current_index and @rows_to_retrieve (ie. 1 and 50) are your paging variables. it’s cleaner and easier to read.

I’ve also tried using SET ROW_COUNT @ROWS_TO_RETRIEVE but doesn’t seem to make much difference.

Using above query and by carefully studying the execution path of the query and modifying/creating indexes and statistics I’ve reached results that are sufficiently satisfactory, hence why i’m making this as the answer. The original goal of retrieving only the required rows in the inner query seems to be not possible yet, if you do find the way please let me know.