[jira] [Created] (ARROW-10134) [C++][Dataset] Add ParquetFileFragment::num_row_groups property

["Joris Van den Bossche (Jira)" <ji...@apache.org>]

Joris Van den Bossche created ARROW-10134:
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             Summary: [C++][Dataset] Add ParquetFileFragment::num_row_groups property
                 Key: ARROW-10134
                 URL: https://issues.apache.org/jira/browse/ARROW-10134
             Project: Apache Arrow
          Issue Type: Improvement
          Components: C++
            Reporter: Joris Van den Bossche
             Fix For: 2.0.0


From https://github.com/dask/dask/pull/6534#issuecomment-699512602, comment by [~rjzamora]:

bq.  it would be great to have access the total row-group count for the fragment from a {{num_row_groups}} attribute (which pyarrow should be able to get without parsing all row-group metadata/statistics - I think?).

One question is: does this attribute correspond to the row groups in the parquet file, or the (subset of) row groups represented by the fragment? 
I expect the second (so if you do SplitByRowGroup, you would get a fragment with num_row_groups==1), but this might be a potential confusing aspect of the attribute.



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