Don’t depend on expression short circuiting in T-SQL (not even with CASE)
March 3, 2011 16 Comments
There are a fair number of blog posts and forum discussions regarding expression short circuiting in T-SQL. Some of the most authoritative posts, like this one, come to the following conclusions: (a) You cannot depend on expression evaluation order for things like “WHERE <expr1> OR <expr2>“, since the optimizer might choose a plan that evaluates the second predicate before the first one. But, (b) order of evaluation of the expressions in a CASE statement is fixed, so you can depend on deterministic short circuit evaluation of a CASE statement. For example, this wouldn’t protect you from a divide-by-zero error:
WHERE (@value = 0) OR ((col1 / @value) = 2)
But the idea is that this variation is functionally-equivalent, and should protect you from the error:
WHERE CASE WHEN (@value = 0) THEN 2 ELSE (col1 / @value) END = 2
Before now that’s the advice I would have offered, too. But I just ran into a situation where a CASE statement does not provide predictable short circuiting. Here’s a simplified repro:
ALTER FUNCTION dbo.test_case_short_circuit (@input INT) RETURNS TABLE AS RETURN ( SELECT calculated_value = CASE WHEN @input <= 0 THEN 0 ELSE LOG10 (@input) END ); GO SELECT * FROM dbo.test_case_short_circuit (0); GO
This fails with this error:
Msg 3623, Level 16, State 1, Line 2
An invalid floating point operation occurred.
The LOG10 function raises this error when its input is 0 or a negative value. In some cases it appears that the plan may still evaluate the expression in the second CASE branch even when it won’t be using the value. This is a case where CASE doesn’t provide deterministic short circuiting.
I want to make sure no one takes away the conclusion that SQL Server doesn’t support expression short circuiting. It definitely does. It’s just that you don’t have explicit control over the order of expression evaluation — even with CASE, apparently. And if you’re going to depend on short circuiting, you need a deterministic order of expression evaluation.
What can you do about it? One option would be to always scrub things so that an error isn’t possible even when the CASE branch’s output won’t be used. For example, using “ELSE LOG10 (CASE WHEN @input <= 0 THEN 1 ELSE @input END)” in the repro script doesn’t change the behavior of the function, but avoids the error. Unfortunately, that’s not so pretty.
UPDATE (4 Mar 2011): To be clear, I’ve used CASE before for its short-circuiting properties, and I don’t intend to go back and revisit all of that old code in light of this example. This seems like an edge case to me. But it’s worth being aware that such edge cases exist if you’re thinking about relying on CASE for short circuiting. The most defensive programming approach would be to write the expression in such a way that it doesn’t require particular short circuiting behavior.
(Cross-posted to my MSDN blog.)
UPDATE (21 Mar 2011): The QO team investigated this and decided that it’s a bug. The problem should be fixed in a future SQL release, but it’s still a small risk on existing releases so keep an eye out for it if you’re using CASE for short circuiting on those releases along with a function like LOG.
UPDATE (10 Jun 2011): The owners of this code have marked this bug as fixed. From their comments, it sounds like you are supposed to be able to rely on deterministic order of expression evaluation for CASE statements. But any SQL release in your hands right now will still be vulnerable to this problem — keep an eye out for the issue as you use CASE for short circuiting.