Does running a SQL Server 2005 database in compatibility level 80 have a negative impact on performance?

Question

Our software must be able to run on SQL Server 2000 and 2005. To simplify development, we\'re running our SQL Server 2005 databases in compatibility level 80. However, dat

Answer 1

Are you using subselects in your queries?

From my experience, a SELECT statement with subselects that runs fine on SQL Server 2000 can crawl on SQL Server 2005 (it can be like 10x slower!).

Make an experiment - re-write one query to eliminate the subselects and see how its performance changes.