SQL EXISTS performance

Question

I understand that SQL EXISTS checks for the existence of rows, however does it evaluate the entire expression? So for example, would something like this:

IF EXISTS (SELECT TOP 1 1 FROM table WITH (NOLOCK))
BEGIN
    ... 
END

Be faster than something like this:

IF EXISTS (SELECT 1 FROM table WITH (NOLOCK))
BEGIN
    ...
END

Answer 1

Both those should run exactly the same. SQL Server takes into account that EXISTS is a short-circuited operation and doesn't evaluate the return result, just checks to see if there IS a return result!

Answer 2

Exists will stop after the first hit because then the expression evaluates to true, so the top(1)-part is unnecessary.

Answer 3

No, it won't.

SQL Server uses TOP in the plan to evaluate EXISTS.

Answer 4

The statements generate identical query plans so there is no difference. The second example is easier to read in my opinion.