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https://issues.apache.org/jira/browse/ARROW-257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15514043#comment-15514043
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Steven Phillips commented on ARROW-257:
---------------------------------------

I don't understand that purpose or benefit of this change. Could you give a 
concrete example of where this would be useful?

> Add a typeids Vector to Union type
> ----------------------------------
>
>                 Key: ARROW-257
>                 URL: https://issues.apache.org/jira/browse/ARROW-257
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: Format
>            Reporter: Julien Le Dem
>            Assignee: Julien Le Dem
>
> {noformat}
> enum UnionMode:int { Sparse, Dense }
> table Union {
>   mode: UnionMode;
>   typeIds: [Int32]; // optional, describes typeid of each child.
> }
> {noformat}
> The idea is to enable providing an id different from the child offset (the 
> default)
> This enables an optimization where we use predefined ids when constructing 
> the type vector of the union but want the children to be only the actually 
> used types.



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