[GitHub] [arrow] rtpsw commented on pull request #13375: ARROW-16823: [C++] Arrow Substrait enhancements for UDF

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rtpsw commented on PR #13375:
URL: https://github.com/apache/arrow/pull/13375#issuecomment-1153940711

   > @rtpsw I did skim through the PR, interesting!. What is required to get a functional test case to evaluate registering a UDF? I went through the JIRA, but it is not clear what is meant by
   > 
   > > registering a function (with an Id) external to the plan
   
   You're right, this isn't trivial. The issue is that `Id` is defined with two members of type `util::string_view`, which point to strings that must be held elsewhere while the plan is in scope. This is satisfied for an `Id` that has these fields pointing to strings parsed from the Substrait plan itself. However, if a user registers a function with an `Id` whose field values are external to the plan, then the strings they point to should be held while the plan is in scope, so it makes sense to keep the strings on the plan; this is what `AddExternalSymbol` is meant for.
   
   I intend to add test cases a bit later. This PR is an extraction from a larger project project I'm working on for end-to-end (Ibis/Ibis-Substrait/PyArrow) support for Python-UDFs.


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