[VariantShredding] Union of partially shredded objects has undefined field order in result object

[Tishj]

parquet-format/VariantShredding.md

Line 307 in 819adce

return VariantObject(metadata, object_fields).union(VariantObject(metadata, value))

The spec says to union the unshredded and shredded values together, in the case of a partially shredded object.
Is there a defined order in which fields should be spliced together?

Without a defined order the result of combining the partially shredded object together will be inconsistent between different implementations, if this is acceptable it should at least be noted in the spec in my opinion.

For example, the object is shredded on fields a and c, the object contains 2 more fields, b and d, in which order should these 4 fields appear in the final result?

[emkornfield]

I think this is covered in the definition of Object:

Object: An unordered collection of string/Variant pairs (i.e. key/value pairs). An object may not contain duplicate keys.

But we should maybe clarify this section as well.

[nssalian]

@emkornfield do you mind assigning this to me? I can put up a PR