It appears that the algebrizer transforms coalesce(a, b) to CASE WHEN a IS NOT NULL THEN a ELSE b ENDbefore optimizer gets a chance to see it. This is testified by the fact a query like "SELECT coalesce(xyz, 0) FROM sys.objects" yields two errormessages for the missing column and not one.When a is a subquery, this becomes CASE WHEN (subquery) IS NOT NULL THEN (subquery) ELSE b ENDThis is unfortunate, because the optimizer does not seem to be able to match the two instances of subquery, why it gets evaluated twice. This can easily be worked around by using isnull instead, but coalesce is in the ANSI standard, and coalesce is also more useful than isnull, since you can have more than two parameters. And more importantly, few users would even suspect that coalesce is bad.In the repro, I use a SELECT query, because it was easier to compose sucha query. In real life, maybe the most important context is UPDATE statementswhere you which to adhere to the ANSI standard. In this case, there are few choices to coalesce.