[jira] [Created] (ARROW-11260) [C++][Dataset] Don't require dictionaries for reading dataset with schema-based Partitioning

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

Joris Van den Bossche created ARROW-11260:
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             Summary: [C++][Dataset] Don't require dictionaries for reading dataset with schema-based Partitioning
                 Key: ARROW-11260
                 URL: https://issues.apache.org/jira/browse/ARROW-11260
             Project: Apache Arrow
          Issue Type: Improvement
          Components: C++
            Reporter: Joris Van den Bossche


As a follow-up on ARROW-10247 (see also https://github.com/apache/arrow/pull/9130#issuecomment-760801124). We currently require the user to pass manually specified dictionary values when reading a dataset with a Partitioning based on a schema with dictionary typed fields. 

In practice that means that the user for example needs to parse the file paths to get all the possible values the partition field can take, while Arrow will then afterwards again do the same to construct the dataset object. 
_Naively_, it seems that it should be possible to let Arrow infer the dictionary _values_, even when providing an explicit schema with a dictionary field for the Partitioning (i.e. so when not letting the partitioning schema itself be inferred from the file paths).

An example use case is when you have a Partitioning schema with both dictionary and non-dictionary fields. When discovering the schema, you can only have all or nothing (all dictionary fields or no dictionary fields).

cc [~bkietz]



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