I can't access the google docs behind a corporate firewall, but the JCS spec
doesn't mention anything about Arrow Union types..
It would probably be best to include Arrow Sparse Union support for something
like:
[
{"user_id": 123},
{"user_id": "Bob"}
]
I can't find the documentation, but I remember reading that arrow union types
are written to parquet as structs, with metadata info so structs can be
converted back into Arrow union types when reading parquet. We probably should
adopt a similar metadata format so the above "user_id" object turns into an
arrow sparse or dense union type. Dense unions have performance issues and
probably should be avoided for a simpler format like json.
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