SQL Server – Retrieving Original Query Based on LSN

I am going to assume that there isn't an index on the date columns, otherwise I think that the query would have been structured differently. If there is, you can probably find a better performing one than this.

The advantage of this query is that it can get all the data in one scan. The disadvantage is that it has to sort the data and join EventEmployee on the entire table. So as always, test with your own situation. This query also assumes that the MAX date is either unique or that equivalent rows would be acceptable.

USE AdventureWorks2012
GO
;
WITH Base AS (
   SELECT 
      TransactionHistory.*
      ,ProductVendor.BusinessEntityID
      ,MAX(CASE WHEN TransactionDate < '2008-08-01' THEN TransactionDate END) 
           OVER (PARTITION BY ProductVendor.BusinessEntityID) AS PreviousVendorTransaction
      ,COUNT(CASE WHEN TransactionDate >= '2008-08-01' THEN 1 END ) 
           OVER (PARTITION BY ProductVendor.BusinessEntityID) AS VendorAfterCutoff
   FROM
      Production.TransactionHistory
      -- Doesn't make the most sense, but I need a repeating relation
      INNER JOIN Purchasing.ProductVendor
         ON TransactionHistory.ProductID = ProductVendor.ProductID
),
Filtered AS (
   SELECT
      *
   FROM
      Base
   WHERE
      Base.TransactionDate >= '2008-08-01'
      OR (TransactionDate = PreviousVendorTransaction AND VendorAfterCutoff > 0)
)
SELECT DISTINCT
   TransactionID
   ,ProductID
   ,ReferenceOrderID
   ,ReferenceOrderLineID
   ,TransactionDate
   ,TransactionType
   ,Quantity
   ,ActualCost
   ,ModifiedDate
FROM
   Filtered

Edit:

Hmm, I think I may have to take back my comment on structuring it differently if there are indexes. The other suggestions that I have are probably fairly minor.

• Make sure the query is using the indexes you're expecting it to. Start and End date to build temp table, end date to drive the previous event loop.
• If the query to build the temp table is doing a lookup on the clustered index, it may be better to hold off and do that as part of the main query.
• Try using a cte instead of a temp table. I think that a cte might be more competitive with the way that the query is structured below.
• If you are returning a lot of events, it might be better to pull out the event table lookup to the main query to give the optimizer the option of doing a merge join.
• I don't see a way of optimizing the previous event lookup short of an indexed view.

Here's a query that combines a few of those ideas.

SELECT
    e.[EventID]
INTO #EventTemp
FROM
    [Events] AS e
WHERE
    ( e.[EventStart] >= @StartDate AND e.[EventStart] <= @EndDate )
    OR ( e.[EventEnd] >= @StartDate AND e.[EventEnd] <= @EndDate )

;
WITH PrevEvent AS (
    SELECT
        EmpPrevEvent.[EventID]
    FROM
    (
        SELECT DISTINCT
            ee.[EmployeeID]
        FROM
            #EventTemp
            INNER JOIN [EventEmployee] AS ee ON
                #EventTemp.[EventID] = ee.[EventID]
    ) AS Emp
    CROSS APPLY (
        SELECT TOP 1
            e.[EventID]
        FROM
            [Events] AS e
            INNER JOIN [EventEmployee] AS ee ON
                e.[EventID] = ee.[EventID]
        WHERE
            ee.[EmployeeID] = Emp.[EmployeeID]
            AND e.[EventEnd] < @StartDate
        ORDER BY 
            e.[EventEnd] DESC
    ) AS EmpPrevEvent
)
SELECT
    e.[EventID],
    e.[EventStart],
    e.[EventEnd],
    e.[EventTypeID]
FROM
    [Events] AS e
WHERE
    e.EventID IN (
        SELECT EventID
        FROM #EventTemp
        UNION
        SELECT EventID
        FROM PrevEvent
    )

For the query plan itself, you can force compilation every time using the following option on your query:

OPTION (RECOMPILE);

However I suspect what is happening is that the second and subsequent executions of the query are pulling the data from memory instead of disk, and of course memory is faster than disk.

If you are trying to test the scenario where this data always comes from disk, there's not really a good way to do that on your production instance without affecting anything else, because you can only drop clean buffers for the entire instance at a time, not for a single database, never mind table. So what you could do perhaps is set up an instance on the same hardware, with just this table, and run the following before every run of the query:

CHECKPOINT;
DBCC DROPCLEANBUFFERS;

However, this does not make a lot of sense, as ideally you will be querying data that is in memory, and you should be optimizing for the best case scenario. You should have some idea of what the worst case scenario is like, but again, this should not be the normal situation, unless you have a very big database and a very small amount of memory (in which case, open the wallet and buy more memory - it's much cheaper than optimizing for the lack of it).