SQL Server IN vs. EXISTS Performance

Question

I'm curious which of the following below would be more efficient?
I've always been a bit cautious about using IN because I believe SQL Server turns the result set into a big IF statement. For a large result set this could result in poor performance. For small results sets, I'm not sure either is preferable. For large result sets, wouldn't EXISTS be more efficient?

WHERE EXISTS (SELECT * FROM Base WHERE bx.BoxID = Base.BoxID AND [Rank] = 2)

vs.

WHERE bx.BoxID IN (SELECT BoxID FROM Base WHERE [Rank = 2])

Answer 1

Off the top of my head and not guaranteed to be correct: I believe the second will be faster in this case.

1. In the first, the correlated subquery will likely cause the subquery to be run for each row.
2. In the second example, the subquery should only run once, since not correlated.
3. In the second example, the IN will short-circuit as soon as it finds a match.

Answer 2

The accepted answer is shortsighted and the question a bit loose in that:

1) Neither explicitly mention whether a covering index is present in the left, right, or both sides.

2) Neither takes into account the size of input left side set and input right side set.
(The question just mentions an overall large result set).

I believe the optimizer is smart enough to convert between "in" vs "exists" when there is a significant cost difference due to (1) and (2), otherwise it may just be used as a hint (e.g. exists to encourage use of an a seekable index on the right side).

Both forms can be converted to join forms internally, have the join order reversed, and run as loop, hash or merge--based on the estimated row counts (left and right) and index existence in left, right, or both sides.

Answer 3

I'd go with EXISTS over IN, see below link:

SQL Server: JOIN vs IN vs EXISTS - the logical difference