When Does NOT IN And NULLable Columns Hurt SQL Server Query Performance?

Posted on by Erik Darling

What This Isn’t

I’m not writing about how if you use NOT IN, and suffer from NULL values in a column you’re comparing, you’ll get an empty result.

That’s easy enough to show:

DECLARE @not_null TABLE ( id INT NOT NULL );
INSERT @not_null ( id )
VALUES ( 1 );

DECLARE @null TABLE ( id INT NULL );
INSERT @null ( id )
VALUES ( 1 );
INSERT @null ( id )
VALUES ( NULL );

SELECT *
FROM   @not_null AS nn
WHERE  nn.id NOT IN ( SELECT n.id FROM @null AS n );

What’s sort of more interesting to me is what happens in execution plans when you’re comparing NULL-able columns that don’t contain any NULLs.

Exampled!

Here’s the first query I want to show you. In the Posts table, the OwnerUserId column is NULLable, but doesn’t have any NULLs in it. The Id column in the Users table is not NULLable — it’s the PK/CX on the table.

SELECT COUNT_BIG(*) AS records
FROM   dbo.Users AS u
WHERE  u.Id NOT IN ( SELECT p.OwnerUserId 
                     FROM dbo.Posts AS p 
					 WHERE p.Score < 0 );

Note that for all these queries, I’ve created these indexes:

CREATE INDEX beavis ON dbo.Users(AccountId);
CREATE INDEX butthead ON dbo.Posts(OwnerUserId, Score);
CREATE INDEX stewart ON dbo.Posts(Score, OwnerUserId);

The important part of the query looks like this:

SQL Server Query Plan
Good thing I have great indexes.

Zooming in to a sorta weird part of the query:

SQL Server Query Plan
I’m in it

The optimizer spins up a Row Count Spool to make sure it’s right about the lack of NULLs. You can see the seek predicate doing so, and finding 0 rows, but taking ~200ms to do it. This time was much worse without those great indexes.

Explicitly Not NULL

If we change our query slightly, we can get a less exotic query plan:

SELECT COUNT_BIG(*) AS records
FROM   dbo.Users AS u
WHERE  u.Id NOT IN (   SELECT p.OwnerUserId
                       FROM   dbo.Posts AS p
                       WHERE  p.Score < 0
                       AND    p.OwnerUserId IS NOT NULL );
SQL Server Query Plan
Not Null’em

Which is a little bit faster overall, at ~350ms vs 550ms.

This is the equivalent of writing your query the way you should have written it in the first place, using NOT EXISTS.

SELECT COUNT_BIG(*) AS records
FROM   dbo.Users AS u
WHERE  NOT EXISTS (   SELECT 1 / 0
                      FROM   dbo.Posts AS p
                      WHERE  p.OwnerUserId = u.Id
                      AND    p.Score < 0 )

Recap

The optimizer is a weird place. It’s the ultimate defensive driver. Dealing with NULLs can be quite odd and difficult.

I generally save IN and NOT IN for when I have short lists of literal values. Likely, EXISTS/NOT EXISTS when values are contained in other tables, or dumping values into a #temp table when there’s a lot of values can be more efficient, if only to make better indexing available.

While IN doesn’t have the same side effects as NOT IN with NULLs, I think using EXISTS instead is just a good habit.

Thanks for reading!

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that and need to solve performance problems quickly.

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