[jira] [Commented] (PARQUET-2249) Parquet spec (parquet.thrift) is inconsistent w.r.t. ColumnIndex + NaNs

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PARQUET-2249?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17721586#comment-17721586 ] 

ASF GitHub Bot commented on PARQUET-2249:
-----------------------------------------

wgtmac commented on PR #196:
URL: https://github.com/apache/parquet-format/pull/196#issuecomment-1543226234

   @JFinis Do you have a plan to revive this?




> 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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