[C++][Dataset] Let datasets be viewable with non-identical schema

[asfimport]

It would be useful to allow some schema unification capability after discovery has completed. For example, if a FileSystemDataset is being wrapped into a UnionDataset with another and their schemas are unifiable then there is no reason we can't create the UnionDataset (rather than emitting an error because the schemas are not identical).

I think this behavior will be most naturally expressed in C++ like so:

virtual Result<Dataset> Dataset::ReplaceSchema(std::shared_ptr<Schema> schema) const = 0;

which will raise an error if the provided schema is not unifiable with the current dataset schema.

If this needs to be extended to non trivial projections then this will probably warrant a separate class, ProjectedDataset or so. Definitely follow up material (if desired)

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++][Dataset] Implement InMemoryDatasetFactory (supercedes)

PRs and other links:

• GitHub Pull Request #6721

_{Note: This issue was originally created as ARROW-8164. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 6721
#6721