SQL Server IN 与 EXISTS 性能

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 result 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

EXISTS will be faster because once the engine has found a hit, it will quit looking as the condition has proved true.

With IN , it will collect all the results from the sub-query before further processing.

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 (eg 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've done some testing on SQL Server 2005 and 2008, and on both the EXISTS and the IN come back with the exact same actual execution plan, as other have stated. The Optimizer is optimal. :)

Something to be aware of though, EXISTS, IN, and JOIN can sometimes return different results if you don't phrase your query just right: http://weblogs.sqlteam.com/mladenp/archive/2007/05/18/60210.aspx

Answer 4

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

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

There is a common misconception that IN behaves equally to EXISTS or JOIN in terms of returned results. This is simply not true.

IN: Returns true if a specified value matches any value in a subquery or a list.

Exists: Returns true if a subquery contains any rows.

Join: Joins 2 resultsets on the joining column.

Blog credit: https://stackoverflow.com/users/31345/mladen-prajdic

Answer 5

There are many misleading answers answers here, including the highly upvoted one (although I don't believe their ops meant harm). The short answer is: These are the same.

There are many keywords in the (T-)SQL language, but in the end, the only thing that really happens on the hardware is the operations as seen in the execution query plan.

The relational (maths theory) operation we do when we invoke [NOT] IN and [NOT] EXISTS is the semi join (anti-join when using NOT ). It is not a coincidence that the corresponding sql-server operations have the same name . There is no operation that mentions IN or EXISTS anywhere - only (anti-)semi joins. Thus, there is no way that a logically-equivalent IN vs EXISTS choice could affect performance because there is one and only way, the (anti)semi join execution operation, to get their results .

An example:

Query 1 ( plan )

select * from dt where dt.customer in (select c.code from customer c where c.active=0)

Query 2 ( plan )

select * from dt where exists (select 1 from customer c where c.code=dt.customer and c.active=0)

Answer 6

在这些情况下，执行计划通常是相同的，但是在您了解优化器如何影响索引等的所有其他方面之前，您真的永远不会知道。

Answer 7

So, IN is not the same as EXISTS nor it will produce the same execution plan.

Usually EXISTS is used in a correlated subquery, that means you will JOIN the EXISTS inner query with your outer query. That will add more steps to produce a result as you need to solve the outer query joins and the inner query joins then match their where clauses to join both.

Usually IN is used without correlating the inner query with the outer query, and that can be solved in only one step (in the best case scenario).

Consider this:

1. If you use IN and the inner query result is millions of rows of distinct values, it will probably perform SLOWER than EXISTS given that the EXISTS query is performant (has the right indexes to join with the outer query).

2. If you use EXISTS and the join with your outer query is complex (takes more time to perform, no suitable indexes) it will slow the query by the number of rows in the outer table, sometimes the estimated time to complete can be in days. If the number of rows is acceptable for your given hardware, or the cardinality of data is correct (for example fewer DISTINCT values in a large data set) IN can perform faster than EXISTS.

3. All of the above will be noted when you have a fair amount of rows on each table (by fair I mean something that exceeds your CPU processing and/or ram thresholds for caching).

So the ANSWER is it DEPENDS. You can write a complex query inside IN or EXISTS, but as a rule of thumb, you should try to use IN with a limited set of distinct values and EXISTS when you have a lot of rows with a lot of distinct values.

The trick is to limit the number of rows to be scanned.

Regards,

MarianoC

Answer 8

To optimize the EXISTS , be very literal; something just has to be there, but you don't actually need any data returned from the correlated sub-query. You're just evaluating a Boolean condition.

So:

WHERE EXISTS (SELECT TOP 1 1 FROM Base WHERE bx.BoxID = Base.BoxID AND [Rank] = 2)

Because the correlated sub-query is RBAR , the first result hit makes the condition true, and it is processed no further.

Answer 9

I know that this is a very old question but I think my answer would add some tips.

I just came across a blog on mssqltips sql exists vs in vs join and it turns out that it is generally the same performance wise.

But the downside of one vs the other are as follows:

1. The in statement has a downside that it can only compare the two tables on one column.

2. The join statement will run on duplicate values, while in and exists will ignore duplicates.

But when you look at the execution time there is no big difference.

The interesting thing is when you create an index on the table, the execution from the join is better.

And I think that join has another upside that it's easier to write and understand especially for newcomers.

Answer 10

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.