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

Just stumbled over this question, and had already found this blog-entry: http://rusanu.com/2009/09/13/on-sql-server-boolean-operator-short-circuit/

The SQL server is free to optimize a query anywhere she sees fit, so in the example given in the blog post, you cannot rely on short-circuiting.

However, a CASE is apparently documented to evaluate in the written order - check the comments of that blog post.

Answer 2

Below a quick and dirty test on SQL Server 2008 R2:

SELECT *
FROM table
WHERE 1=0
AND (function call to complex operation)

This returns immediately with no records. Kind of short circuit behavior was present.

Then tried this:

SELECT *
FROM table
WHERE (a field from table) < 0
AND (function call to complex operation)

knowing no record would satisfy this condition:

(a field from table) < 0

This took several seconds, indicating the short circuit behavior was not there any more and the complex operation was being evaluated for every record.

Hope this helps guys.