Same columns, by name and type, in the same order. Nullability is ignored, as the engine ignores it when concatenating relations.
Same columns by name, type and nullability, in the same order: what source
schema drift detection compares, unlike equals().
The column with that name, or undefined
The column names, in order
The type of the column with that name, or undefined
The ordered, immutable columns of a relation. Column names are unique, as the engine requires.