[jira] [Assigned] (ARROW-8065) [C++][Dataset] Untangle Dataset, Fragment and ScanOptions

["Francois Saint-Jacques (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-8065?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Francois Saint-Jacques reassigned ARROW-8065:
---------------------------------------------

    Assignee: Francois Saint-Jacques

> [C++][Dataset] Untangle Dataset, Fragment and ScanOptions
> ---------------------------------------------------------
>
>                 Key: ARROW-8065
>                 URL: https://issues.apache.org/jira/browse/ARROW-8065
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Francois Saint-Jacques
>            Assignee: Francois Saint-Jacques
>            Priority: Major
>              Labels: dataset
>
> Currently: a fragment is a product of a scan; it is a lazy collection of scan tasks corresponding to a data source which is logically singular (like a single file, a single row group, ...). It would be more useful if instead a fragment were the direct object of a scan; one scans a fragment (or a collection of fragments):
>  # Remove {{ScanOptions}} from Fragment's properties and move it into {{Fragment::Scan}} parameters.
>  # Remove {{ScanOptions}} from {{Dataset::GetFragments}}. We can provide an overload to support predicate pushdown in FileSystemDataset and UnionDataset {{Dataset::GetFragments(std::shared_ptr<Expression> predicate)}}.
>  # Expose lazy accessor to Fragment::physical_schema()
>  # Consolidate ScanOptions and ScanContext
> This will lessen the cognitive dissonance between fragments and files since fragments will no longer include references to scan properties.



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