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https://issues.apache.org/jira/browse/ARROW-10640?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17237243#comment-17237243
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Joris Van den Bossche commented on ARROW-10640:
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And in SQL, this is related to a "CASE WHEN" clause:
https://www.postgresqltutorial.com/postgresql-case/ (although here you can
provide multiple boolean conditions, and it's also using expressions instead of
actual boolean masks, but that's something to optimize only when there is a
query engine)
> [C++] A "where" kernel to combine two arrays based on a mask
> ------------------------------------------------------------
>
> Key: ARROW-10640
> URL: https://issues.apache.org/jira/browse/ARROW-10640
> Project: Apache Arrow
> Issue Type: New Feature
> Components: C++
> Reporter: Joris Van den Bossche
> Priority: Major
>
> (from discussion in ARROW-9489 with [~maartenbreddels])
> A general "where" kernel like {{np.where}}
> (https://numpy.org/doc/stable/reference/generated/numpy.where.html) seems a
> generally useful kernel to have, and could also help mimicking some other
> python (setitem-like) operations.
> The concrete use case in ARROW-9489 is to basically do a
> {{fill_null(array[string], array[string])}} which could be expressed as
> {{where(is_null(arr), arr2, arr)}}.
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