[jira] [Commented] (ARROW-8733) [C++][Dataset][Python] ParquetFileFragment should provide access to parquet FileMetadata

["Francois Saint-Jacques (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-8733?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17101909#comment-17101909 ] 

Francois Saint-Jacques commented on ARROW-8733:
-----------------------------------------------

We could expose this yes. In ARROW-8062, I'm constructing the ParquetFileFragment without holding the original FileMetaData for mutltiple reasons, but more importantly because it may or not map to what the real physical FileMetaData holds. I think that it would be ill conceived to create a fake FileMetaData constructed from the `_metadata`. I plan to expose the statistics probably via shared_ptr<Expression>.

There is various details to flesh out regarding this, especially the one where a fragment only contains a strict subset of the row groups.

What about serialization for dask? That's another potential issue.

> [C++][Dataset][Python] ParquetFileFragment should provide access to parquet FileMetadata
> ----------------------------------------------------------------------------------------
>
>                 Key: ARROW-8733
>                 URL: https://issues.apache.org/jira/browse/ARROW-8733
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++, Python
>            Reporter: Joris Van den Bossche
>            Priority: Major
>              Labels: dataset
>             Fix For: 1.0.0
>
>
> Related to ARROW-8062 (as there we will also need a way to expose the global FileMetadata). But independently, it would be useful to get access to the FileMetadata on each {{ParquetFileFragment}} (eg to get access to the statistics).
> This would be relatively simple to code on the Python/R side, since we have access to the file path, and could read the metadata from the file backing the fragment, and return this as a FileMetadata object. 
> I am wondering if we want to integrate this with ARROW-8062, since when the fragments were created from a {{_metadata}} file, a {{ParquetFileFragment.metadata}} attribute would not need to read it from the parquet file in this case, but from the global metadata (at least for eg the row group data).
> Another question: what for a ParquetFileFragment that maps to a single row group?



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