Is the SQL WHERE clause short-circuit evaluated?

Question

Are boolean expressions in SQL WHERE clauses short-circuit evaluated ?

For example:

SELECT * 
FROM Table t 
WHERE @key IS NULL OR (@key IS NOT NULL          


        

Answer 1

I don't believe that short circuiting in SQL Server (2005) is guaranteed. SQL Server runs your query through its optimization algorithm that takes into account a lot of things (indexes, statistics, table size, resources, etc) to come up with an effective execution plan. After this evaluation, you can't say for sure that your short circuit logic is guaranteed.

I ran into the same question myself sometime ago and my research really did not give me a definitive answer. You may write a small query to give you a sense of proof that it works but can you be sure that as the load on your database increases, the tables grow to be bigger, and things get optimized and changed in the database, that conclusion will hold. I could not and therefore erred on the side of caution and used CASE in WHERE clause to ensure short circuit.