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Posted to jira@arrow.apache.org by "Wes McKinney (Jira)" <ji...@apache.org> on 2020/06/29 04:10:00 UTC
[jira] [Resolved] (ARROW-9009) [C++][Dataset] ARROW:schema should
be removed from schema's metadata when reading Parquet files
[ https://issues.apache.org/jira/browse/ARROW-9009?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Wes McKinney resolved ARROW-9009.
---------------------------------
Resolution: Fixed
Resolved in ARROW-8980
> [C++][Dataset] ARROW:schema should be removed from schema's metadata when reading Parquet files
> -----------------------------------------------------------------------------------------------
>
> Key: ARROW-9009
> URL: https://issues.apache.org/jira/browse/ARROW-9009
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++
> Reporter: Joris Van den Bossche
> Assignee: Wes McKinney
> Priority: Major
> Labels: dataset
> Fix For: 1.0.0
>
>
> When reading a parquet file (which was written by Arrow) with the datasets API, it preserves the "ARROW:schema" field in the metadata:
> {code:python}
> import pyarrow as pa
> import pyarrow.parquet as pq
> import pyarrow.dataset as ds
> table = pa.table({'a': [1, 2, 3]})
> pq.write_table(table, "test.parquet")
> dataset = ds.dataset("test.parquet", format="parquet")
> {code}
> {code}
> In [7]: dataset.schema
> Out[7]:
> a: int64
> -- field metadata --
> PARQUET:field_id: '1'
> -- schema metadata --
> ARROW:schema: '/////3gAAAAQAAAAAAAKAAwABgAFAAgACgAAAAABAwAMAAAACAAIAAAABA' + 114
> In [8]: dataset.to_table().schema
> Out[8]:
> a: int64
> -- field metadata --
> PARQUET:field_id: '1'
> -- schema metadata --
> ARROW:schema: '/////3gAAAAQAAAAAAAKAAwABgAFAAgACgAAAAABAwAMAAAACAAIAAAABA' + 114
> {code}
> while when reading with the `parquet` module reader, we do not preserve this metadata:
> {code}
> In [9]: pq.read_table("test.parquet").schema
> Out[9]:
> a: int64
> -- field metadata --
> PARQUET:field_id: '1'
> {code}
> Since the "ARROW:schema" information is used to properly reconstruct the Arrow schema from the ParquetSchema, it is no longer needed once you already have the arrow schema, so it's probably not needed to keep it as metadata in the arrow schema.
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