Ensure custom functions can write data

Closes #1808

lib/src/sql/function.rs

@@ -86,6 +86,10 @@ impl Function {
 			_ => unreachable!(),
 		}
 	}
+	/// Check if this function is a custom function
+	pub fn is_custom(&self) -> bool {
+		matches!(self, Self::Custom(_, _))
+	}
 	/// Check if this function is a rolling function
 	pub fn is_rolling(&self) -> bool {
 		match self {

lib/src/sql/value/value.rs

@@ -1417,7 +1417,7 @@ impl Value {
 			Value::Block(v) => v.writeable(),
 			Value::Array(v) => v.iter().any(Value::writeable),
 			Value::Object(v) => v.iter().any(|(_, v)| v.writeable()),
-			Value::Function(v) => v.args().iter().any(Value::writeable),
+			Value::Function(v) => v.is_custom() || v.args().iter().any(Value::writeable),
 			Value::Subquery(v) => v.writeable(),
 			Value::Expression(v) => v.l.writeable() || v.r.writeable(),
 			_ => false,