jayzhan211 commented on code in PR #12308:
URL: https://github.com/apache/datafusion/pull/12308#discussion_r1743596400


##########
datafusion/functions/src/core/coalesce.rs:
##########
@@ -60,12 +60,16 @@ impl ScalarUDFImpl for CoalesceFunc {
     }
 
     fn return_type(&self, arg_types: &[DataType]) -> Result<DataType> {
-        Ok(arg_types[0].clone())
+        Ok(arg_types
+            .iter()
+            .find_or_first(|d| !d.is_null())
+            .unwrap()
+            .clone())
     }
 
-    // If all the element in coalesce is non-null, the result is non-null
+    // If all the elements in coalesce are non-null, the result is non-null
     fn is_nullable(&self, args: &[Expr], schema: &dyn ExprSchema) -> bool {
-        args.iter().any(|e| e.nullable(schema).ok().unwrap_or(true))
+        args.iter().all(|e| e.nullable(schema).ok().unwrap_or(true))

Review Comment:
   I don't think you need to modify the `coerce_types` in coalesce.
   
   `coerce_types` is the function that coerce the types, paired with 
`Signature::user_defined`.
   
   The actual issue here is that we doesn't return the coerced types in the 
code I mentioned above
   



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