update comments and link to follow-on issue

Author: andygrove

datafusion/expr/src/binary_rule.rs

@@ -72,6 +72,12 @@ pub fn binary_operator_data_type(
 
 /// Coercion rules for all binary operators. Returns the output type
 /// of applying `op` to an argument of `lhs_type` and `rhs_type`.
+///
+/// TODO this function is trying to serve two purposes at once; it determines the result type
+/// of the binary operation and also determines how the inputs can be coerced but this
+/// results in inconsistencies in some cases (particular around date + interval)
+///
+/// Tracking issue is https://github.com/apache/arrow-datafusion/issues/3419
 pub fn coerce_types(
     lhs_type: &DataType,
     op: &Operator,

datafusion/optimizer/src/type_coercion.rs

@@ -101,8 +101,7 @@ impl ExprRewriter for TypeCoercionRewriter {
                         DataType::Date32 | DataType::Date64 | DataType::Timestamp(_, _),
                         &DataType::Interval(_),
                     ) => {
-                        // Arrow `can_cast_types` says we cannot cast an Interval to
-                        // Date32/Date64/Timestamp, which contradicts DataFusion's `coerce_types`
+                        // this is a workaround for https://github.com/apache/arrow-datafusion/issues/3419
                         Ok(expr.clone())
                     }
                     _ => {