Murmur hash gives different hash values for same input value about cardinalityestimation HOT 6 CLOSED

Maybe this happens because the Murmur3 is not thread safe i.e. if you call it from different threads you could have a wrong hash.

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Thanks Sergey for the fix!

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Thanks, will check the fix, however I see the same when I use the cardinality estimation in a single thread. I think it is might because I add "long" numbers. I check adding 10000000 random numbers between 10000000 and 10000100, so only 100 different numbers. And when I compare with a hashset the hashset actually contains 100 numbers and the estimator estimates more (which it is not expected since we have a directcount). The strange thing is that it only occurs sometimes although I initiate my random number generator with a fixed seed and the code is not multithreaded (unless the cardinality estimator adds multi threading).

from cardinalityestimation.

Ok, my test was wrong since I used testmethods for both the multithreaded and singlethreaded scenario. And, since the test-methods run in parallel and the Murmur3-hash is static, even the singlethreaded testmethod actually tested the multithreaded scenario.. :-S.
Running a much larger test on the single-threaded scenario confirmed the observation that the issue was the thread-safety.
Thanks for fixing!

Arthur

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Hi Oron,

Are you planing to publish these changes to nuget?

Thanks,
Sergey

On Fri, Aug 5, 2016 at 5:49 AM, Oron Navon [email protected] wrote:

Closed #12 #12.

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Hi Sergey,Thanks for the reminder :) Published as 1.2.1.
Thanks,Oron

Date: Fri, 19 Aug 2016 15:32:49 -0700
From: [email protected]
To: [email protected]
CC: [email protected]; [email protected]
Subject: Re: [Microsoft/CardinalityEstimation] Murmur hash gives different hash values for same input value (#12)

Hi Oron,

Are you planing to publish these changes to nuget?

Thanks,

Sergey

On Fri, Aug 5, 2016 at 5:49 AM, Oron Navon [email protected] wrote:

Closed #12 #12.

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