Right now we supporting updating a subset of columns with merge insert. However, we left when_not_matched_insert_all() as TODO. (see: #2639). Users should be able to provide a subset of columns in the merge insert query, and have the missing columns filled in with null. (Relatedly, we might also consider something similar with append.)

We left this as TODO in large part because we haven't seriously approached null values. When inserting a subset of columns, the missing columns should be filled with nulls. The default V1 format doesn't universally support nulls, so it would not have worked well. Now that V2 is the default, this will make more sense. We should decide again whether we should allow missing (null) fields in fragments, or require that there be all null columns in the data files. We have a vestigial comment in our proto files right now: