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https://issues.apache.org/jira/browse/ARROW-4139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16823146#comment-16823146
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Wes McKinney commented on ARROW-4139:
-------------------------------------
I added this to my list of things to look at early this week
> [Python] Cast Parquet column statistics to unicode if UTF8 ConvertedType is
> set
> -------------------------------------------------------------------------------
>
> Key: ARROW-4139
> URL: https://issues.apache.org/jira/browse/ARROW-4139
> Project: Apache Arrow
> Issue Type: Bug
> Components: Python
> Reporter: Matthew Rocklin
> Priority: Minor
> Labels: parquet, python
> Fix For: 0.14.0
>
>
> When writing Pandas data to Parquet format and reading it back again I find
> that that statistics of text columns are stored as byte arrays rather than as
> unicode text.
> I'm not sure if this is a bug in Arrow, PyArrow, or just in my understanding
> of how best to manage statistics. (I'd be quite happy to learn that it was
> the latter).
> Here is a minimal example
> {code:python}
> import pandas as pd
> df = pd.DataFrame({'x': ['a']})
> df.to_parquet('df.parquet')
> import pyarrow.parquet as pq
> pf = pq.ParquetDataset('df.parquet')
> piece = pf.pieces[0]
> rg = piece.row_group(0)
> md = piece.get_metadata(pq.ParquetFile)
> rg = md.row_group(0)
> c = rg.column(0)
> >>> c
> <pyarrow._parquet.ColumnChunkMetaData object at 0x7fd1a377c238>
> file_offset: 63
> file_path:
> physical_type: BYTE_ARRAY
> num_values: 1
> path_in_schema: x
> is_stats_set: True
> statistics:
> <pyarrow._parquet.RowGroupStatistics object at 0x7fd1a37d4418>
> has_min_max: True
> min: b'a'
> max: b'a'
> null_count: 0
> distinct_count: 0
> num_values: 1
> physical_type: BYTE_ARRAY
> compression: SNAPPY
> encodings: ('PLAIN_DICTIONARY', 'PLAIN', 'RLE')
> has_dictionary_page: True
> dictionary_page_offset: 4
> data_page_offset: 25
> total_compressed_size: 59
> total_uncompressed_size: 55
> >>> type(c.statistics.min)
> bytes
> {code}
> My guess is that we would want to store a logical type in the statistics like
> UNICODE, though I don't have enough experience with Parquet data types to
> know if this is a good idea or possible.
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