Performance of SQL Server 2005 Query

Question

-------------------- this takes 4 secs to execute (with 2000 000 rows) WHY?---------------------

DECLARE @AccountId INT 
DECLARE @Max INT 
DECLARE @MailingLi         


        

Answer 1

The second query has to process ONLY 2000 records. Point.

The first has to process ALL records to find the maximum.

Top 2000 does not get you the highest 2000, it gets you the first 2000 of the result set - in any order.

if yo uwant to change them to be identical, the second should read

TOP 1

and then order by anp_Subscriber.Id descending (plus fast first option).

Answer 2

Add OPTION (RECOMPILE) to the end of the query.

SQL Server doesn't "sniff" the values of the variables so you will be getting a plan based on guessed statistics rather than one tailored for the actual variable values.

Answer 3

One possible item to check is whether the MailingListId and AccountId fields in the tables are of type INT. If, for example, the types are BIGINT, the query optimizer will often not use the index on those fields. When you explicitly define the values instead of using variables, the values are implicitly converted to the proper type.

Make sure the types match.