Hello, Does Pinot support defining a computed field (metric) based on a field that does not appear in the schema but exists in the ingestion message? This is a realtime table if that makes a difference.

Yes this is supported

So the transformationFunction can receive as a parameter a field that does not exist in the schema? Is there documentation on this use-case?

Yes. If you have a function like bar="reverse(foo)", where bar is in the Pinot schema, but foo is not in the Pinot schema, it will work if foo is present in the source record. Pinot will look for all arguments to the function in the row