ASF GitHub Bot commented on PARQUET-1225:

boroknagyz commented on issue #444: PARQUET-1225: NaN values may lead to 
incorrect filtering under certaiā€¦
URL: https://github.com/apache/parquet-cpp/pull/444#issuecomment-367726337
   Thanks for applying the changes! To me it looks good generally.
   The Update() and UpdateSpaced() functions share some common code parts (not 
necessarily introduced by this PR, e.g. the last if statements of the 
functions), maybe worth a little refactoring.

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> NaN values may lead to incorrect filtering under certain circumstances
> ----------------------------------------------------------------------
>                 Key: PARQUET-1225
>                 URL: https://issues.apache.org/jira/browse/PARQUET-1225
>             Project: Parquet
>          Issue Type: Task
>          Components: parquet-cpp
>            Reporter: Zoltan Ivanfi
>            Assignee: Deepak Majeti
>            Priority: Major
> _This JIRA describes a generic problem with floating point comparisons that 
> *most probably* affects parquet-cpp. It is known to affect Impala and by 
> taking a quick look at the parquet-cpp code it seems to affect parquet-cpp as 
> well, but it has not yet been confirmed in practice._
> For comparing float and double values for min/max stats, parquet-cpp uses the 
> C++ less-than operator (<) that returns false for comparisons involving a 
> NaN. This means that while garthering statistics, if a NaN is the smallest 
> value encountered so far (which happens to be the case after reading the 
> first value if that value is NaN), no other value can ever replace it, since 
> < will always be false. On the other hand, if NaN is not the first value, it 
> won't affect the min value. So the min value depends on the order of elements.
> If looking for specific values while reading back the data, the NaN value may 
> lead to row groups being incorrectly discarded in spite of having matching 
> rows. For details, please see the Impala bug IMPALA-6527.

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