Proper use of the ColumnChunk `file_path` attribute

[Richard Zamora <rz...@nvidia.com>]

I’d like to solicit some feedback on the use of the `file_path` attribute for ColumnChunk metadata in Parquet.  How exactly is this attribute used in practice for both single-file and distributed datasets?

More specifically: Is it bad form to set the `file_path` value in footer metadata when the data is stored in the same file?  Should the value only be set in the `_metadat` file, or in cases where the actual column-chunk data is stored in a different location?  My intuition is that the answer to both of these questions is “yes,”  but any feedback/details from people with strong parquet experience is very welcome :)

Note that the context for these questions is an ongoing discussion about the necessary metadata API in `arrow.parquet` (e.g. https://github.com/apache/arrow/pull/4361 and https://issues.apache.org/jira/browse/ARROW-5349?page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel&focusedCommentId=16845670#comment-16845670)

Thanks for your help!
-Rick

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