ASF GitHub Bot commented on PARQUET-2249:

zhongyujiang commented on PR #196:
URL: https://github.com/apache/parquet-format/pull/196#issuecomment-1481237476

   > Thus, to solve the problem of only-NaN pages, the comments in the spec are 
extended to mandate the following behavior:
   > Once a writer writes the nan_count/nan_counts fields, they have to:
   > never write NaN into min/max if there are non-NaN non-Null values and
   > always write min=max=NaN if the only non-null values in a page are NaN
   > A reader observing that nan_count/nan_counts field was written can then 
rely on that if min or max are NaN, then both have to be NaN and this means 
that the only non-NULL values are NaN.
   Instead of writing min and max as NaN when there are only NaN values and 
then let the reader to check whether min max  NaN are credible by evaluating 
whether naNCounts is empty, wouldn't it be much simpler if we just left the 
evaluation of isNaN and notNaN to the reader?
   A reader can always conclude a page / column is all NaN when value count of 
the field == NaN count of the filed (when valueCounts and naNCounts both 
exists), this's Iceberg's current way of [evaluating 
  Is there anything wrong with doing this in Parquet?

> Parquet spec (parquet.thrift) is inconsistent w.r.t. ColumnIndex + NaNs
> -----------------------------------------------------------------------
>                 Key: PARQUET-2249
>                 URL: https://issues.apache.org/jira/browse/PARQUET-2249
>             Project: Parquet
>          Issue Type: Bug
>          Components: parquet-format
>            Reporter: Jan Finis
>            Priority: Major
> Currently, the specification of {{ColumnIndex}} in {{parquet.thrift}} is 
> inconsistent, leading to cases where it is impossible to create a parquet 
> file that is conforming to the spec.
> The problem is with double/float columns if a page contains only NaN values. 
> The spec mentions that NaN values should not be included in min/max bounds, 
> so a page consisting of only NaN values has no defined min/max bound. To 
> quote the spec:
> {noformat}
>    *     When writing statistics the following rules should be followed:
>    *     - NaNs should not be written to min or max statistics 
> fields.{noformat}
> However, the comments in the ColumnIndex on the null_pages member states the 
> following:
> {noformat}
> struct ColumnIndex {
>   /**
>    * A list of Boolean values to determine the validity of the corresponding
>    * min and max values. If true, a page contains only null values, and 
> writers
>    * have to set the corresponding entries in min_values and max_values to
>    * byte[0], so that all lists have the same length. If false, the
>    * corresponding entries in min_values and max_values must be valid.
>    */
>   1: required list<bool> null_pages{noformat}
> For a page with only NaNs, we now have a problem. The page definitly does 
> *not* only contain null values, so {{null_pages}} should be {{false}} for 
> this page. However, in this case the spec requires valid min/max values in 
> {{min_values}} and {{max_values}} for this page. As the only value in the 
> page is NaN, the only valid min/max value we could enter here is NaN, but as 
> mentioned before, NaNs should never be written to min/max values.
> Thus, no writer can currently create a parquet file that conforms to this 
> specification as soon as there is a only-NaN column and column indexes are to 
> be written.
> I see three possible solutions:
> 1. A page consisting only of NaNs (or a mixture of NaNs and nulls) has it's 
> null_pages entry set to {*}true{*}.
> 2. A page consisting of only NaNs (or a mixture of NaNs and nulls) has 
> {{byte[0]}} as min/max, even though the null_pages entry is set to 
> {*}false{*}.
> 3. A page consisting of only NaNs (or a mixture of NaNs and nulls) does have 
> NaN as min & max in the column index.
> None of the solutions is perfect. But I guess solution 3. is the best of 
> them. It gives us valid min/max bounds, makes null_pages compatible with 
> this, and gives us a way to determine only-Nan pages (min=max=NaN).
> As a general note: I would say that it is a shortcoming that Parquet doesn't 
> track NaN counts. E.g., Iceberg does track NaN counts and therefore doesn't 
> have this inconsistency. In a future version, NaN counts could be introduced, 
> but that doesn't help for backward compatibility, so we do need a solution 
> for now.
> Any of the solutions is better than the current situation where engines 
> writing such a page cannot write a conforming parquet file and will randomly 
> pick any of the solutions.
> Thus, my suggestion would be to update parquet.thrift to use solution 3. 
> I.e., rewrite the comments saying that NaNs shouldn't be included in min/max 
> bounds by adding a clause stating that "if a page contains only NaNs or a 
> mixture of NaNs and NULLs, then NaN should be written as min & max".

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