Add doc comment next to INTERNAL_GROUPING_ID

datafusion/expr/src/logical_plan/plan.rs

@@ -3066,6 +3066,23 @@ impl Aggregate {
         }
     }
 
+    /// Internal column used when the aggregation is a grouping set.
+    ///
+    /// This column contains a bitmask where each bit represents a grouping
+    /// expression. The least significant bit corresponds to the rightmost
+    /// grouping expression. A bit value of 0 indicates that the corresponding
+    /// column is included in the grouping set, while a value of 1 means it is excluded.
+    ///
+    /// For example, for the grouping expressions CUBE(a, b), the grouping ID
+    /// column will have the following values:
+    ///     0b00: Both `a` and `b` are included
+    ///     0b01: `b` is excluded
+    ///     0b10: `a` is excluded
+    ///     0b11: Both `a` and `b` are excluded
+    ///
+    /// This internal column is necessary because excluded columns are replaced
+    /// with `NULL` values. To handle these cases correctly, we must distinguish
+    /// between an actual `NULL` value in a column and a column being excluded from the set.
     pub const INTERNAL_GROUPING_ID: &'static str = "__grouping_id";
 }