Fix up test and comment

Author: alamb

datafusion/src/scalar.rs

@@ -34,7 +34,6 @@ use std::{convert::TryFrom, fmt, iter::repeat, sync::Arc};
 
 /// Represents a dynamically typed, nullable single value.
 /// This is the single-valued counter-part of arrow’s `Array`.
-//#[derive(PartialEq, Eq, PartialOrd, Ord, Hash, Clone)]
 #[derive(Clone)]
 pub enum ScalarValue {
     /// true or false value
@@ -1528,8 +1527,9 @@ mod tests {
 
     #[test]
     fn size_of_group_by_scalar() {
-        // ensure the size is not too large (??)
-        // TODO what are the implications of this being 64 bytes rather than 16 bytes?
-        assert_eq!(std::mem::size_of::<ScalarValue>(), 16);
+        // Since hash aggregations uses the size of a ScalarValue,
+        // making it larger means the grouping operation requires more
+        // memory per distinct value.
+        assert_eq!(std::mem::size_of::<ScalarValue>(), 64);
     }
 }