[C++] Calculate output type from aggregate to convert arrow aggregate to substrait

[asfimport]

I am adding support for mapping to/from Arrow aggregates and Substrait aggregates in ARROW-15582. However, the Arrow-Substrait direction is currently blocked because the Substrait plan needs to know the output type of an aggregate and there is no easy way to determine that from the Arrow information we have.

We should be able to get this information from the function registry but the conversion routines do not have access to the function registry that I can tell. I'm not sure if the best solution is to pass the function registry into ToProto or to add the output type to the aggregate in Arrow.

Reporter: Weston Pace / @westonpace
Assignee: Vibhatha Lakmal Abeykoon / @vibhatha

_{Note: This issue was originally created as ARROW-17239. Please see the migration documentation for further details.}

[asfimport]

Vibhatha Lakmal Abeykoon / @vibhatha:
This was also brought up when doing the roundtrip plans for Acero to Substrait. This is a PR under review for emit feature. I think I need to update that piece of code with the following. 

SubstraitCall new interface contains the output_type attribute, may be we can use this and aggregate function name to create the output field. 

Do we need else where other than ToProtos/Emit for aggregate?

 

[asfimport]

Vibhatha Lakmal Abeykoon / @vibhatha:
cc @westonpace  

[asfimport]

Weston Pace / @westonpace:
SubstraitCall seems like the right place to store the output type. However, the challenge is figuring out what the output type is supposed to be and not where to put it.

[github-actions]

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