[jira] [Commented] (ARROW-4516) Error while creating a ParquetDataset on a path without `_common_dataset` but with an empty `_tempfile`

["yogesh garg (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-4516?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16763905#comment-16763905 ] 

yogesh garg commented on ARROW-4516:
------------------------------------

Originally posted on https://github.com/apache/arrow/issues/3594

> Error while creating a ParquetDataset on a path without `_common_dataset` but with an empty `_tempfile`
> -------------------------------------------------------------------------------------------------------
>
>                 Key: ARROW-4516
>                 URL: https://issues.apache.org/jira/browse/ARROW-4516
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: Python
>    Affects Versions: 0.12.0
>            Reporter: yogesh garg
>            Priority: Major
>
> I suspect that there's an error in this line of code:
> https://github.com/apache/arrow/blob/master/python/pyarrow/parquet.py#L926
> While validating schema in the initialisation of a {{ParquetDataset}}, we assume that if {{_common_metadata}} file does not exist, the schema should be inferred from the first piece of that dataset. The first piece, in my experience, could refer to a file named with an underscore, that does not necessarily have to contain the schema, and could be an empty file, e.g. {{_tempfile}}.
> {code:bash}
> /tmp/pq/
> ├── part1.parquet
> └── _tempfile
> {code}
> This behavior is allowed by the parquet specification, and we should probably ignore such pieces.
> On a cursory look, we could do either of the following.
> 1. Choose the first piece with path that does not start with "_"
> 2. Sort pieces by name, but put all the "_" pieces later while making the manifest. https://github.com/apache/arrow/blob/master/python/pyarrow/parquet.py#L729
> 3. Silently exclude all the files starting with "_" here, but this will need to be tested: https://github.com/apache/arrow/blob/master/python/pyarrow/parquet.py#L770



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