Generics, Traits, and Extension Boundaries

DataFusion uses traits where third parties or runtime query shape require an open set: ExecutionPlan, PhysicalExpr, TableProvider, QueryPlanner, optimizer rules, scalar functions, and MemoryPool. These commonly appear as Arc<dyn Trait + Send + Sync> because one plan must contain unlike nodes.

Generics are strongest inside reusable implementations. Stream adapters are generic over their inner stream or produced item; expression kernels can be generic over Arrow array types; tree utilities accept closures that preserve the visitor operation without creating a new public object hierarchy.

Associated types and trait bounds tie values together when callers benefit from that relationship. Type erasure happens at the collection boundary:

concrete scan/filter/aggregate structs
             ↓ coercion
Vec<Arc<dyn ExecutionPlan>> and Arc<dyn PhysicalExpr>
             ↓ execution
Pin<Box<dyn RecordBatchStream + Send>>

This keeps user extensions possible and compile times manageable. Making the entire plan a deeply nested generic type would encode a runtime SQL statement in a compile-time type the caller cannot name.