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https://issues.apache.org/jira/browse/ARROW-6179?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16903949#comment-16903949
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Micah Kornfield commented on ARROW-6179:
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Ok, personally I would like to leave the current behavior as at least the
default. One example of the usage on non registration of extension types is
the BQ storage read API uses it to mark fields that don't have a one to one
correspondence with built in arrow types (geography and datetime). In the
future someone could choose to write custom extension types but in the meantime
they don't require special handling and flow through without any problem when
converting to pandas.
> [C++] ExtensionType subclass for "unknown" types?
> -------------------------------------------------
>
> Key: ARROW-6179
> URL: https://issues.apache.org/jira/browse/ARROW-6179
> Project: Apache Arrow
> Issue Type: Improvement
> Reporter: Joris Van den Bossche
> Priority: Major
>
> In C++, when receiving IPC with extension type metadata for a type that is
> unknown (the name is not registered), we currently fall back to returning the
> "raw" storage array. The custom metadata (extension name and metadata) is
> still available in the Field metadata.
> Alternatively, we could also have a generic {{ExtensionType}} class that can
> hold such "unknown" extension type (eg {{UnknowExtensionType}} or
> {{GenericExtensionType}}), keeping the extension name and metadata in the
> Array's type.
> This could be a single class where several instances can be created given a
> storage type, extension name and optionally extension metadata. It would be a
> way to have an unregistered extension type.
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