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https://issues.apache.org/jira/browse/ARROW-10130?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Joris Van den Bossche reassigned ARROW-10130:
---------------------------------------------
Assignee: Joris Van den Bossche
> [C++][Dataset] ParquetFileFragment::SplitByRowGroup does not preserve
> "complete_metadata" status
> ------------------------------------------------------------------------------------------------
>
> Key: ARROW-10130
> URL: https://issues.apache.org/jira/browse/ARROW-10130
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++
> Reporter: Joris Van den Bossche
> Assignee: Joris Van den Bossche
> Priority: Major
> Fix For: 2.0.0
>
>
> Splitting a ParquetFileFragment in multiple fragments per row group
> ({{SplitByRowGroup}}) calls {{EnsureCompleteMetadata}} initially, but then
> the created fragments afterwards don't have the {{has_complete_metadata_}}
> property set. This means that when calling {{EnsureCompleteMetadata}} on the
> splitted fragments, it will read/parse the metadata again, instead of using
> the cached ones (which are already present).
> Small example to illustrate:
> {code:python}
> In [1]: import pyarrow.dataset as ds
> In [2]: dataset =
> ds.parquet_dataset("nyc-taxi-data/dask-partitioned/_metadata",
> partitioning="hive")
> In [3]: rg_fragments = [rg for frag in dataset.get_fragments() for rg in
> frag.split_by_row_group()]
> In [4]: len(rg_fragments)
> Out[4]: 4520
> # row group fragments actually have statistics
> In [7]: rg_fragments[0].row_groups[0].statistics
> Out[7]:
> {'vendor_id': {'min': '1', 'max': '4'},
> 'pickup_at': {'min': datetime.datetime(2009, 1, 1, 0, 5, 51),
> 'max': datetime.datetime(2018, 12, 26, 14, 48, 54)},
> ...
> # but calling ensure_complete_metadata still takes a lot of time the first
> call
> In [8]: %time _ = [fr.ensure_complete_metadata() for fr in rg_fragments]
> CPU times: user 1.72 s, sys: 203 ms, total: 1.92 s
> Wall time: 1.9 s
> In [9]: %time _ = [fr.ensure_complete_metadata() for fr in rg_fragments]
> CPU times: user 1.34 ms, sys: 0 ns, total: 1.34 ms
> Wall time: 1.35 ms
> {code}
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