Risks of NOLOCK, part 2

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Risks of NOLOCK, part 2

Hugo Kornelis

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November 2, 2015 at 10:53 pm

#399267

Comments posted to this topic are about the item Risks of NOLOCK, part 2
Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
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November 2, 2015 at 10:55 pm

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November 3, 2015 at 5:21 am

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call.copse SSCoach Points: 18043 More actions · Answer 1

call.copse

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November 3, 2015 at 2:56 am

#1837294

Thanks Hugo! Nice question.

Carlo Romagnano SSC-Insane Points: 22662 More actions · Answer 2

Carlo Romagnano

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Points: 22662

November 3, 2015 at 3:31 am

#1837303

Excellent!

edwardwill SSCertifiable Points: 5843 More actions · Answer 3

edwardwill

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Points: 5843

November 3, 2015 at 4:40 am

#1837312

One of the options is missing a checkbox

Ed Wagner SSC Guru Points: 287026 More actions · Answer 4

edwardwill (11/3/2015)
One of the options is missing a checkbox

Yes. The third checkbox looks like it should be split into two separate checkboxes.

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Ed Wagner SSC Guru Points: 287026 More actions · Answer 5

Stewart "Arturius" Campbell (11/3/2015)
Ed Wagner (11/3/2015)
edwardwill (11/3/2015)
One of the options is missing a checkbox
Yes. The third checkbox looks like it should be split into two separate checkboxes.
You'll notice the second half of that line matches the line beneath it.

Ahhh. I didn't even think to compare it to the others. Nice catch.

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michal.lisinski SSCertifiable Points: 6933 More actions · Answer 6

Very good question Hugo, but I'm still not sure that "Nolock CAN NOT cause the query to not return committed rows" is the wrong answer.

Could anybody clarify me this point better?

Thanks in advance,

Mike

below86 SSChampion Points: 11532 More actions · Answer 7

Getting closer, got 2 of 4 yesterday and 3 of 4 today. Thanks for the questions, learning something new everyday.

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Don't fear failure, fear regret.

Hugo Kornelis SSC Guru Points: 64790 More actions · Answer 8

Ed Wagner (11/3/2015)
Stewart "Arturius" Campbell (11/3/2015)
Ed Wagner (11/3/2015)
edwardwill (11/3/2015)
One of the options is missing a checkbox
Yes. The third checkbox looks like it should be split into two separate checkboxes.
You'll notice the second half of that line matches the line beneath it.
Ahhh. I didn't even think to compare it to the others. Nice catch.

It appears that I made a copy/paste error when submitting the question. I pasted the third option in the third input box, and then apparently pasted the fourth option there as well before pasting it in the fourth box.

Those who have seen the interface for submitting a QotD will understand this. The boxes don't have room to show the entire answer, and in this case the answers are so similar that I didn't notice it while scrolling through the boxes. My apologies.

Steve, if you see this - any chance that you can get this fixed? I don't think it will cause people to miss the question, but it does annoy me. 🙂

Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

Hugo Kornelis SSC Guru Points: 64790 More actions · Answer 9

michal.lisinski (11/3/2015)
Very good question Hugo, but I'm still not sure that "Nolock CAN NOT cause the query to not return committed rows" is the wrong answer.
Could anybody clarify me this point better?

I hope that this is not due to the double negation. I was debating on how to phrase this. The alternative would have been "Nolock CAN NOT cause the query to skip committed rows", which I at this time actually prefer over what I submitted.

Anyway, the explanation is the same as for returning the same row twice. Connection 1 starts a scan and processes pages 1 to 1000. At that time, connection 2 makes a change that causes a page split on page 1500. SQL Server sees that page 500 is empty, so it decides to allocate the new page there, then moves half the rows from page 1500 to page 500. The scan from connection 1 continues; once it reaches page 1500 it will only see the rows that are left there after the page split, not the ones that have been moved. And since i has already passed page 500, it will not see them at any time during the rest of the scan.

I hope this helps!

Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

George Vobr SSCrazy Eights Points: 9989 More actions · Answer 10

Thank you Hugo, refined questions very well prepared. It gave me a hard time, but it helped me this article:

http://blogs.sqlsentry.com/aaronbertrand/bad-habits-nolock-everywhere/

Kim Crosser SSCommitted Points: 1763 More actions · Answer 11

Hi Hugo - I got all four of these (surprise, surprise)... 😎

Like any tool, you should understand what it does before you use it...

Hany Helmy SSChampion Points: 13921 More actions · Answer 12

Thanx Hugo 4 those 2 very useful questions, NoLock=NoDoubt 😉

Thanks & Best Regards,
Hany Helmy
SQL Server Database Consultant