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https://issues.apache.org/jira/browse/ARROW-16339?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17538269#comment-17538269
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Antoine Pitrou commented on ARROW-16339:
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Q1, Q2: agreed with [~emkornfield]
Q3: no idea about this
Q4: it would sound reasonable to merge metadata, with Arrow keys taking
precedence over Parquet keys (if Parquet defines {{"key": "foo"}} and Arrow
defines {{"key": "bar"}}, keep only {{"key": "bar"}}).
> [C++][Parquet] Parquet FileMetaData key_value_metadata not always mapped to
> Arrow Schema metadata
> -------------------------------------------------------------------------------------------------
>
> Key: ARROW-16339
> URL: https://issues.apache.org/jira/browse/ARROW-16339
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++, Parquet, Python
> Reporter: Joris Van den Bossche
> Priority: Critical
> Fix For: 9.0.0
>
>
> Context: I ran into this issue when reading Parquet files created by GDAL
> (using the Arrow C++ APIs, [https://github.com/OSGeo/gdal/pull/5477]), which
> writes files that have custom key_value_metadata, but without storing
> ARROW:schema in those metadata (cc [~paleolimbot]
> —
> Both in reading and writing files, I expected that we would map Arrow
> {{Schema::metadata}} with Parquet {{{}FileMetaData::key_value_metadata{}}}.
> But apparently this doesn't (always) happen out of the box, and only happens
> through the "ARROW:schema" field (which stores the original Arrow schema, and
> thus the metadata stored in this schema).
> For example, when writing a Table with schema metadata, this is not stored
> directly in the Parquet FileMetaData (code below is using branch from
> ARROW-16337 to have the {{store_schema}} keyword):
> {code:python}
> import pyarrow as pa
> import pyarrow.parquet as pq
> table = pa.table({'a': [1, 2, 3]}, metadata={"key": "value"})
> pq.write_table(table, "test_metadata_with_arrow_schema.parquet")
> pq.write_table(table, "test_metadata_without_arrow_schema.parquet",
> store_schema=False)
> # original schema has metadata
> >>> table.schema
> a: int64
> -- schema metadata --
> key: 'value'
> # reading back only has the metadata in case we stored ARROW:schema
> >>> pq.read_table("test_metadata_with_arrow_schema.parquet").schema
> a: int64
> -- schema metadata --
> key: 'value'
> # and not if ARROW:schema is absent
> >>> pq.read_table("test_metadata_without_arrow_schema.parquet").schema
> a: int64
> {code}
> It seems that if we store the ARROW:schema, we _also_ store the schema
> metadata separately. But if {{store_schema}} is False, we also stop writing
> those metadata (not fully sure if this is the intended behaviour, and that's
> the reason for the above output):
> {code:python}
> # when storing the ARROW:schema, we ALSO store key:value metadata
> >>> pq.read_metadata("test_metadata_with_arrow_schema.parquet").metadata
> {b'ARROW:schema': b'/////7AAAAAQAAAAAAAKAA4ABgAFAA...',
> b'key': b'value'}
> # when not storing the schema, we also don't store the key:value
> >>> pq.read_metadata("test_metadata_without_arrow_schema.parquet").metadata
> >>> is None
> True
> {code}
> On the reading side, it seems that we generally do read custom key/value
> metadata into schema metadata. We don't have the pyarrow APIs at the moment
> to create such a file (given the above), but with a small patch I could
> create such a file:
> {code:python}
> # a Parquet file with ParquetFileMetaData::metadata that ONLY has a custom key
> >>> pq.read_metadata("test_metadata_without_arrow_schema2.parquet").metadata
> {b'key': b'value'}
> # this metadata is now correctly mapped to the Arrow schema metadata
> >>> pq.read_schema("test_metadata_without_arrow_schema2.parquet")
> a: int64
> -- schema metadata --
> key: 'value'
> {code}
> But if you have a file that has both custom key/value metadata and an
> "ARROW:schema" key, we actually ignore the custom keys, and only look at the
> "ARROW:schema" one.
> This was the case that I ran into with GDAL, where I have a file with both
> keys, but where the custom "geo" key is not also included in the serialized
> arrow schema in the "ARROW:schema" key:
> {code:python}
> # includes both keys in the Parquet file
> >>> pq.read_metadata("test_gdal.parquet").metadata
> {b'geo': b'{"version":"0.1.0","...',
> b'ARROW:schema': b'/////3gBAAAQ...'}
> # the "geo" key is lost in the Arrow schema
> >>> pq.read_table("test_gdal.parquet").schema.metadata is None
> True
> {code}
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