[jira] [Commented] (ARROW-11647) [C++][Compute] CastFromNull does not use preallocated buffers

["Apache Arrow JIRA Bot (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-11647?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17617231#comment-17617231 ] 

Apache Arrow JIRA Bot commented on ARROW-11647:
-----------------------------------------------

This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned per [project policy|https://arrow.apache.org/docs/dev/developers/bug_reports.html#issue-assignment]. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

> [C++][Compute] CastFromNull does not use preallocated buffers
> -------------------------------------------------------------
>
>                 Key: ARROW-11647
>                 URL: https://issues.apache.org/jira/browse/ARROW-11647
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Ben Kietzman
>            Assignee: Eduardo Ponce
>            Priority: Minor
>
> When casting from null, currently new buffers are allocated for every batch of the computation. This is wasteful as for simple types data buffers are preallocated and the null bitmap is handled separately; CastFromNull need do no work at all (unless we decide to explicitly zero the data buffer). For varlength out types the offsets buffer is preallocated and should be zeroed, for struct types preallocation is not implemented (but should be as simple as preallocating each child array).



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