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https://issues.apache.org/jira/browse/ARROW-12873?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17355203#comment-17355203
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Ben Kietzman commented on ARROW-12873:
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> You mention "since they may not originate from the arrow library". Do you
> have an example of that?
libarrow_dataset is currently separate from libarrow
> It seems pretty straightforward that an exec batch would have a partition
> expression associated with it.
That seems reasonable, and since Expression is in the compute:: namespace these
days we can attach {{Expression ExecBatch::guarantee}} without much trouble.
> One thing I am wondering about, which is maybe a bit academic, is what are
> the rules of preserving tags that operators (ExecNodes) must follow.
That's an excellent point; it's not at all clear what nodes should do with
batch-level tags if the node doesn't have a straightforward 1-in-1-out process.
For now in https://github.com/apache/arrow/pull/10397 I'm pursuing a workaround
of augmenting scanned batches with virtual columns which encode their fragment
and batch indices, which can be used to reorder batches in a ToTable operation.
Perhaps something similar could be more generally applicable to the problem of
tagging batches: users who need to tag batches can augment with virtual columns
containing keys into a table which they maintain.
> [C++][Compute] Support tagging ExecBatches with arbitrary extra information
> ---------------------------------------------------------------------------
>
> Key: ARROW-12873
> URL: https://issues.apache.org/jira/browse/ARROW-12873
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Ben Kietzman
> Priority: Major
>
> Ideally, ExecBatches could be tagged with arbitrary optional objects for
> tracing purposes and to transmit execution hints from one ExecNode to another.
> These should *not* be explicit members like ExecBatch::selection_vector is,
> since they may not originate from the arrow library. For an example within
> the arrow project: {{libarrow_dataset}} will be used to produce ScanNodes and
> a WriteNodes and it's useful to tag scanned batches with their {{Fragment}}
> of origin. However adding {{ExecBatch::fragment}} would result in a cyclic
> dependency.
> To facilitate this tagging capability, we would need a type erased container
> something like
> {code}
> struct AnySet {
> void* Get(tag_t tag);
> void Set(tag_t tag, void* value, FnOnce<void(void*)> destructor);
> };
> {code}
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