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https://issues.apache.org/jira/browse/ARROW-11763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17290777#comment-17290777
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ARF commented on ARROW-11763:
-----------------------------
[~westonpace] I will modify ARROW-11678 to focus on the coercing without
warning to user angle. I am afraid the C++ code of arrow is far beyond my
skills. I looked at it a while back and found it positively daunting to even
navigate the code.
> [C++] Dict index type ALWAYS gets coerced to int32 when saving to parquet
> -------------------------------------------------------------------------
>
> Key: ARROW-11763
> URL: https://issues.apache.org/jira/browse/ARROW-11763
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++
> Affects Versions: 3.0.0
> Reporter: ARF
> Priority: Major
>
> On saving a pyarrow Dictionary-type column to parquet, any non-int32 index
> gets coerced to an int32 index without warning:
> {code:python}
> import pyarrow as pa
> from pyarrow import parquet as pq
> schema = pa.schema({
> 'foo': pa.dictionary(pa.int8(), pa.string(), ordered=False),
> })
> def make_trivial_dict_array(dict_type, value, size):
> return
> table = pa.Table.from_pydict({
> 'foo': pa.DictionaryArray.from_arrays(
> pa.nulls(1, schema.field('foo').type.index_type).fill_null(0),
> ['bar'])
> })
> pq.write_table(table, 'test_dict_int8.parquet', version='2.0',
> data_page_version='2.0')
> print(f"dict index type before saving to parquet:
> {table.schema.field('foo').type.index_type}")
> del table
> table = pq.read_table('test_dict_int8.parquet')
> print(f"dict index type after saving to parquet:
> {table.schema.field('foo').type.index_type}")
> {code}
> Output:
> {code:java}
> dict index type before saving to parquet: int8
> dict index type after saving to parquet: int32
> {code}
> While this is surprising for smaller index types, coercing an int64 index to
> an int32 index without warning the user seems like asking for trouble.
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