Coerce Array inner types

datafusion/expr-common/src/type_coercion/binary.rs

@@ -1138,27 +1138,44 @@ fn numeric_string_coercion(lhs_type: &DataType, rhs_type: &DataType) -> Option<D
     }
 }
 
+fn coerce_list_children(lhs_field: &FieldRef, rhs_field: &FieldRef) -> Option<FieldRef> {
+    Some(Arc::new(
+        Arc::unwrap_or_clone(Arc::clone(lhs_field)).with_data_type(comparison_coercion(
+            lhs_field.data_type(),
+            rhs_field.data_type(),
+        )?),
+    ))
+}
+
 /// Coercion rules for list types.
 fn list_coercion(lhs_type: &DataType, rhs_type: &DataType) -> Option<DataType> {
     use arrow::datatypes::DataType::*;
     match (lhs_type, rhs_type) {
-        (List(_), List(_)) => Some(lhs_type.clone()),
-        (LargeList(_), List(_)) => Some(lhs_type.clone()),
-        (List(_), LargeList(_)) => Some(rhs_type.clone()),
-        (LargeList(_), LargeList(_)) => Some(lhs_type.clone()),
-        (List(_), FixedSizeList(_, _)) => Some(lhs_type.clone()),
-        (FixedSizeList(_, _), List(_)) => Some(rhs_type.clone()),
+        (
+            LargeList(lhs_field),
+            List(rhs_field) | LargeList(rhs_field) | FixedSizeList(rhs_field, _),
+        )
+        | (FixedSizeList(lhs_field, _) | List(lhs_field), LargeList(rhs_field)) => {
+            Some(LargeList(coerce_list_children(lhs_field, rhs_field)?))
+        }
+
+        (List(lhs_field), List(rhs_field) | FixedSizeList(rhs_field, _))
+        | (FixedSizeList(lhs_field, _), List(rhs_field)) => {
+            Some(List(coerce_list_children(lhs_field, rhs_field)?))
+        }
+
         // Coerce to the left side FixedSizeList type if the list lengths are the same,
         // otherwise coerce to list with the left type for dynamic length
-        (FixedSizeList(lf, ls), FixedSizeList(_, rs)) => {
+        (FixedSizeList(lhs_field, ls), FixedSizeList(rhs_field, rs)) => {
             if ls == rs {
-                Some(lhs_type.clone())
+                Some(FixedSizeList(
+                    coerce_list_children(lhs_field, rhs_field)?,
+                    *rs,
+                ))
             } else {
-                Some(List(Arc::clone(lf)))
+                Some(List(coerce_list_children(lhs_field, rhs_field)?))
             }
         }
-        (LargeList(_), FixedSizeList(_, _)) => Some(lhs_type.clone()),
-        (FixedSizeList(_, _), LargeList(_)) => Some(rhs_type.clone()),
         _ => None,
     }
 }
@@ -2105,6 +2122,19 @@ mod tests {
             DataType::List(Arc::clone(&inner_field))
         );
 
+        // Negative test: inner_timestamp_field and inner_field are not compatible because their inner types are not compatible
+        let inner_timestamp_field = Arc::new(Field::new(
+            "item",
+            DataType::Timestamp(TimeUnit::Microsecond, None),
+            true,
+        ));
+        let result_type = get_input_types(
+            &DataType::List(Arc::clone(&inner_field)),
+            &Operator::Eq,
+            &DataType::List(Arc::clone(&inner_timestamp_field)),
+        );
+        assert!(result_type.is_err());
+
         // TODO add other data type
         Ok(())
     }

datafusion/sqllogictest/test_files/union.slt

@@ -761,3 +761,7 @@ SELECT NULL WHERE FALSE;
 ----
 0.5
 1
+
+# Test Union of List Types. Issue: https://github.com/apache/datafusion/issues/12291
+query error DataFusion error: type_coercion\ncaused by\nError during planning: Incompatible inputs for Union: Previous inputs were  of type List(.*), but got incompatible type List(.*) on column 'x'
+SELECT make_array(2) x UNION ALL SELECT make_array(now()) x;