Does SQL Server read all of a COALESCE function even if the first argument is not NULL?

How about this one?

DECLARE @i INT = 1;
SELECT CASE WHEN @i = 1 THEN 1 ELSE MIN(1/0) END;

Result:

Msg 8134, Level 16, State 1, Line 2
Divide by zero error encountered.

There are trivial workarounds of course, but the point is still that CASE does not always guarantee left-to-right evaluation / short-circuiting.

EDIT just an addendum, while I agree that these are edge cases, that most of the time you can rely on left-to-right evaluation and short-circuiting, and that these are bugs that contradict the documentation and will probably eventually be fixed (this isn't definite - see the follow-up conversation on Bart Duncan's blog post to see why), I have to disagree when folks say that something is always true even if there is a single edge case that disproves it. If Itzik and others can find solitary bugs like this, it makes it at least in the realm of possibility that there are other bugs as well. And since we don't know the rest of the OP's query, we can't say for certain that he will rely on this short-circuiting but end up being bitten by it. So to me, the safer answer is:

While you can usually rely on CASE to evaluate left-to-right and short-circuit, as described in the documentation, it is not accurate to say that you can always do so. There are two demonstrated cases on this page where it is not true, and neither bug has been fixed in any publicly available version of SQL Server.

EDIT here is another case (I need to stop doing that) where a CASE expression does not evaluate in the order you would expect, even though no aggregates are involved:

http://connect.microsoft.com/SQLServer/feedback/details/780132/

My view on this is that the documentation makes it reasonably clear that the intention is that CASE should short-circuit. As Aaron mentions, there have been several cases (ha!) where this has been shown to not always be true.

So far, all these have been acknowledged as bugs and fixed - though not necessarily in a version of SQL Server you can buy and patch up today (the constant-folding bug has not yet made it to a Cumulative Update AFAIK). The newest potential bug - originally reported by Itzik Ben-Gan - has yet to be investigated (either Aaron or I will add it to Connect shortly).

Related to the original question, there are other issues with CASE (and therefore COALESCE) where side-effecting functions or sub-queries are used. Consider:

SELECT COALESCE((SELECT CASE WHEN RAND() <= 0.5 THEN 999 END), 999);
SELECT ISNULL((SELECT CASE WHEN RAND() <= 0.5 THEN 999 END), 999);

The COALESCE form often returns NULL, more details at https://connect.microsoft.com/SQLServer/feedback/details/546437/coalesce-subquery-1-may-return-null

The demonstrated issues with optimizer transforms and common-expression-tracking mean that it is impossible to guarantee that CASE will short-circuit in all circumstances. I can conceive of cases where it might not even be possible to predict the behaviour by inspecting the public show plan output, though I don't have a repro for that today.

In summary, I think you can be reasonably confident that CASE will short-circuit in general (particularly if a reasonably-skilled person inspects the execution plan, and that execution plan is 'enforced' with a plan guide or hints) but if you need an absolute guarantee, you have to write SQL that does not include the expression at all.

Not a hugely satisfactory state of affairs, I guess.

I've come across another case where CASE / COALESCE do not short circuit. The following TVF will raise a PK violation if passed 1 as a parameter.

CREATE FUNCTION F (@P INT)
RETURNS @T TABLE (
  C INT PRIMARY KEY)
AS
  BEGIN
      INSERT INTO @T
      VALUES      (1),
                  (@P)

      RETURN
  END

If called as follows

DECLARE @Number INT = 1

SELECT COALESCE(@Number, (SELECT number
                          FROM   master..spt_values
                          WHERE  type = 'P'
                                 AND number = @Number), 
                         (SELECT TOP (1)  C
                          FROM   F(@Number))) 

Or as

DECLARE @Number INT = 1

SELECT CASE
         WHEN @Number = 1 THEN @Number
         ELSE (SELECT TOP (1) C
               FROM   F(@Number))
       END 

Both give the result

Violation of PRIMARY KEY constraint 'PK_F_3BD019A800551192'. Cannot insert duplicate key in object 'dbo.@T'. The duplicate key value is (1).

showing that the SELECT (or at least the table variable population) is still carried out and raises an error even though that branch of the statement should never be reached. The plan for the COALESCE version is below.

This rewrite of the query appears to avoid the issue

SELECT COALESCE(Number, (SELECT number
                          FROM   master..spt_values
                          WHERE  type = 'P'
                                 AND number = Number), 
                         (SELECT TOP (1)  C
                          FROM   F(Number))) 
FROM (VALUES(1)) V(Number)   

Which gives plan

I just wanted to mention a strategy you may not have considered. It may not be a match here, but it does come in handy sometimes. See if this modification gives you any better performance:

SELECT COALESCE(c.FirstName
            ,(SELECT TOP 1 b.FirstName
              FROM TableA a 
              JOIN TableB b ON .....
              WHERE C.FirstName IS NULL) -- this is the changed part
            )

Another way to do it could be this (basically equivalent, but allows you to access more columns from the other query if necessary):

SELECT COALESCE(c.FirstName, x.FirstName)
FROM
   TableC c
   OUTER APPLY (
      SELECT TOP 1 b.FirstName
      FROM
         TableA a 
         JOIN TableB b ON ...
      WHERE
         c.FirstName IS NULL -- the important part
   ) x

Basically this is a technique of "hard" joining tables but including the condition on when any rows at all should be JOINed. In my experience this has really helped execution plans at times.

Neither COALESCE nor CASE are guaranteed to short-circuit in SQl Server. In fact, it can be demonstrated that they do not.

My example is pulled from here. Turn on Include Actual Execution Plan, and run this query:

DECLARE @test INT
SET @test = 1
SELECT COALESCE(@test, (SELECT COUNT(*) FROM sysobjects))

Then look at the execution plan.

Update: My example is flawed, see @Aaron's answer for a working example.

No, it would not. It would only run when c.FirstName is NULL.

However, you should try it yourself. Experiment. You said your subquery is lengthy. Benchmark. Draw your own conclusions on this.

@Aaron answer on the sub-query being run is more complete.

However, I still think you should rework your query and use LEFT JOIN. Most of the time, sub queries can be removed by reworking your query to use LEFT JOINs.

The problem with using sub queries is that your overall statement will run slower because the sub query is ran for each row in main query's result set.