[Python] Return parquet statistics min/max as values instead of strings

[asfimport]

Currently min and max column statistics are returned as formatted strings of the physical type. This makes using them in python a bit tricky, as the strings need to be parsed as the proper logical type. Observe:

In [20]: import pandas as pd

In [21]: df = pd.DataFrame({'a': [1, 2, 3],
    ...:                    'b': ['a', 'b', 'c'],
    ...:                    'c': [pd.Timestamp('1991-01-01')]*3})
    ...:

In [22]: df.to_parquet('temp.parquet', engine='pyarrow')

In [23]: from pyarrow import parquet as pq

In [24]: f = pq.ParquetFile('temp.parquet')

In [25]: rg = f.metadata.row_group(0)

In [26]: rg.column(0).statistics.min  # string instead of integer
Out[26]: '1'

In [27]: rg.column(1).statistics.min  # weird space added after value due to formatter
Out[27]: 'a '

In [28]: rg.column(2).statistics.min  # formatted as physical type (int) instead of logical (datetime)
Out[28]: '662688000000'

Since the type information is known, it should be possible to convert these to arrow values instead of strings.

Reporter: Jim Crist / @jcrist
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #1698

_{Note: This issue was originally created as ARROW-1982. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
This seems easy enough to fix. Marked for 0.9.0

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1698
#1698