Rails Migration DSL and Relational Database Rollback Safety: Which Schema Changes Require Explicit Reversible Logic?

Rails migrations expose a DSL intended for reversible schema evolution, yet the boundary between the framework's automatic rollback inference and the underlying relational database's DDL semantics remains an unresolved design concern. Teams need a predictable contract for when db:rollback can safely invert a schema change without manual intervention.

The core uncertainty centers on operations that alter existing column types or introduce constrained defaults. While Rails can infer an inverse for simple additions or removals, change_column migrations and database-specific handling of dropped columns do not share a uniform rollback contract across PostgreSQL and MySQL. The DSL's reversible block offers a manual escape hatch, but determining when it is mandatory versus optional is not always obvious from documented behavior alone.

Which change_table operations are guaranteed to invert automatically across supported relational databases, and under what conditions does Rails silently skip an irreversible change instead of raising an error? How should teams model the integration boundary between the migration DSL and database-specific DDL semantics to ensure db:rollback remains safe after type-altering migrations?