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

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

yogesh garg created ARROW-4516:
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             Summary: 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


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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