Why schema compare requires the same engine
Structural diff needs a shared dialect. Cross-engine work is Conversion Studio, not compare.
Schema comparison in Migratrix expects matching engines on both sides. A PostgreSQL catalog and an Oracle catalog do not share constraint, type, and identifier rules that a line-oriented diff can apply honestly. The product therefore refuses to pretend Oracle versus PostgreSQL is a schema sync.
Moving structure between engines is Conversion Studio: analyse the source, draft a target model, override mapping, then run. After cutover, compare PostgreSQL to PostgreSQL (or MySQL to MySQL) to keep the new estate aligned.
MongoDB has no structural schema reader, so compare there is data-only. Collections still participate in Conversion Studio as source and target.