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https://issues.apache.org/jira/browse/ARROW-257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15420729#comment-15420729
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Wes McKinney commented on ARROW-257:
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So if I understand correctly, support we had a union of 50 types, but only 5 of
them actually occur in the data, then the typeIds would indicate the indices of
the observed child types. That makes sense to me.
> 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
>
> {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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