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https://issues.apache.org/jira/browse/ARROW-6536?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16928017#comment-16928017
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Antoine Pitrou commented on ARROW-6536:
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Because you may want to pass a subset of the CSV file's columns.
To emulate passing a schema, you can probably set both {{column_names}} and
{{column_types}}. Though it probably deserves checking whether it does what you
expect.
> [C++] CSV reader accept schema
> ------------------------------
>
> Key: ARROW-6536
> URL: https://issues.apache.org/jira/browse/ARROW-6536
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Neal Richardson
> Priority: Major
> Labels: csv, dataset
>
> The CSV reader lets you specify {{column_types}}, but this is an
> {{unordered_map}} of column name and type. Why not accept a Schema instead?
> Isn't that essentially an ordered map? Seems that if you took a Schema, some
> of the validation of what's being passed in would already have been handled.
> Plus, I suspect that the Datasets project will want to do even more with
> passing a Schema (e.g. selecting a subset of columns).
> Thoughts [~pitrou] [~fsaintjacques] [~bkietz]?
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