How Functions Can Make Performance Tuning SQL Server Queries Harder

Sensational

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I’ve posted quite a bit about how cached plans can be misleading.

I’m gonna switch that up and talk about how an actual plan can be misleading, too.

In plans that include calling a muti-statement table valued function, no operator logs the time spent in the function.

Here’s an example:

SELECT TOP (100)
     p.Id AS [Post Link],
     vs.up,
     vs.down
FROM dbo.VoteStats() AS vs --The function
JOIN dbo.Posts AS p
    ON vs.postid = p.Id
WHERE vs.down > vs.up_multiplier
AND   p.CommunityOwnedDate IS NULL
AND   p.ClosedDate IS NULL
ORDER BY vs.up DESC

When I run the query, it drags on for 30-ish seconds, but the plan says that it only ran for about 2.7 seconds.

But there it is in Query Time Stats! 29 seconds. What gives?

Estimations

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If we look at the estimated plan for the function, we can see quite a thick arrow pointing to the table variable we populate for our results.

That process is all part of the query, but it doesn’t show up in any of the operators. It really should.

More specifically, I think it should show up right here.

Thanks for reading!

Going Further

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If this is the kind of SQL Server stuff you love learning about: these days I have AI do the annoying stuff for me. My new video course teaches you to work the same way, and I teach whole teams in a one-day workshop. Someone still has to know enough SQL Server to keep the robots in check, and that’s what my training is for.