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https://issues.apache.org/jira/browse/PARQUET-2249?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17738941#comment-17738941
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ASF GitHub Bot commented on PARQUET-2249:
-----------------------------------------
gszadovszky commented on code in PR #196:
URL: https://github.com/apache/parquet-format/pull/196#discussion_r1247575939
##########
src/main/thrift/parquet.thrift:
##########
@@ -966,6 +985,23 @@ struct ColumnIndex {
/** A list containing the number of null values for each page **/
5: optional list<i64> null_counts
+
+ /**
+ * A list of Boolean values to determine pages that contain only NaNs. Only
+ * present for columns of type FLOAT and DOUBLE. If true, all non-null
+ * values in a page are NaN. Writers are suggested to set the corresponding
+ * entries in min_values and max_values to NaN, so that all lists have the
same
+ * length and contain valid values. If false, then either all values in the
+ * page are null or there is at least one non-null non-NaN value in the page.
+ * As readers are supposed to ignore all NaN values in bounds, legacy readers
+ * who do not consider nan_pages yet are still able to use the column index
+ * but are not able to skip only-NaN pages.
+ */
+ 6: optional list<bool> nan_pages
Review Comment:
@mapleFU, I did not think about any specific implementation. (TBH, I only
have experince with parquet-mr.) This is mentioned in the PR description.
Maybe, we do not have any implementations as such.
@JFinis, I agree we should not care about the potential systems already
writing NaN values into column indexes. Also agree that writing NaN values to
min/max is risky for existing systems. So we need to write non-NaN valid values
to min/max for all-NaN pages. (And of course mark them with either `nan_pages`
or `value_counts`.)
The more we narrow the range the higher the chance the page will be dropped
during filtering which is good because we should not search for NaN values
based on the spec anyway. What do you think about `[-Inf, -Inf]`? The worst
case is we will read the page of all NaN values instead of dropping. In this
very case we would not drop it for `< x` like cases. (This turned out to be the
rephrasing and summary of your previous comments. :smile: )
> 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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