[ARROW-2079] [Python][C++] Possibly use `_common_metadata` for schema if `_metadata` isn't available

• Type: Improvement
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Component/s: Python
• Labels: dataset dataset-parquet-read parquet
• Assignee: Unassigned
• Reporter: Jim Crist
• Created: 01/Feb/18 21:55
• Updated: 04/Jun/20 13:27

Description

Currently pyarrow's parquet writer only writes `_common_metadata` and not `_metadata`. From what I understand these are intended to contain the dataset schema but not any row group information.

 

A few (possibly naive) questions:

 

1. In the `_init_` for `ParquetDataset`, the following lines exist:

if self.metadata_path is not None:
    with self.fs.open(self.metadata_path) as f:
        self.common_metadata = ParquetFile(f).metadata
else:
    self.common_metadata = None

I believe this should use `common_metadata_path` instead of `metadata_path`, as the latter is never written by `pyarrow`, and is given by the `_metadata` file instead of `_common_metadata` (as seemingly intended?).

 

2. In `validate_schemas` I believe an option should exist for using the schema from `_common_metadata` instead of `_metadata`, as pyarrow currently only writes the former, and as far as I can tell `_common_metadata` does include all the schema information needed.

 

Perhaps the logic in `validate_schemas` could be ported over to:

 

if self.schema is not None:
    pass  # schema explicitly provided
elif self.metadata is not None:
    self.schema = self.metadata.schema
elif self.common_metadata is not None:
    self.schema = self.common_metadata.schema
else:
    self.schema = self.pieces[0].get_metadata(open_file).schema

If these changes are valid, I'd be happy to submit a PR. It's not 100% clear to me the difference between `_common_metadata` and `_metadata`, but I believe the schema in both should be the same. Figured I'd open this for discussion.

Issue Links

• is related to: ARROW-2209 [Python] Partition columns are not correctly loaded in schema of ParquetDataset
• is related to: ARROW-8062 [C++][Dataset] Parquet Dataset factory from a _metadata/_common_metadata file
• relates to: ARROW-8446 [Python][Dataset] Detect and use _metadata file in a list of file paths