[jira] [Created] (ARROW-8802) [C++][Dataset] Schema metadata are lost when reading a subset of columns

[Joris Van den Bossche (Jira)]

Joris Van den Bossche created ARROW-8802:
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             Summary: [C++][Dataset] Schema metadata are lost when reading a 
subset of columns
                 Key: ARROW-8802
                 URL: https://issues.apache.org/jira/browse/ARROW-8802
             Project: Apache Arrow
          Issue Type: Bug
          Components: C++
            Reporter: Joris Van den Bossche


Python example:

{code}
import pandas as pd     
import pyarrow.dataset as ds                                                    
                                                                                
                                          

df = pd.DataFrame({'a': [1, 2, 3]})  
df.to_parquet("test_metadata.parquet")  

dataset = ds.dataset("test_metadata.parquet")                                   
                                                                                
                                          
{code}

gives:
{code}
>>> dataset.to_table().schema 
a: int64
  -- field metadata --
  PARQUET:field_id: '1'
-- schema metadata --
pandas: '{"index_columns": [{"kind": "range", "name": null, "start": 0, "' + 397
ARROW:schema: '/////4ACAAAQAAAAAAAKAA4ABgAFAAgACgAAAAABAwAQAAAAAAAKAAwAAA' + 806

>>> dataset.to_table(columns=['a']).schema 
a: int64
  -- field metadata --
  PARQUET:field_id: '1'
{code}

So when specifying a subset of the columns, the additional metadata entries are 
lost (while those can still be informative, eg for conversion to pandas)



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