feat: optimize and unparse grouping

datafusion/functions-aggregate/src/grouping.rs

@@ -35,25 +35,25 @@ make_udaf_expr_and_func!(
     Grouping,
     grouping,
     expression,
-    "Returns 1 if the data is aggregated across the specified column or 0 for not aggregated in the result set.",
+    "Returns the level of grouping, equals to (grouping(c1) << (n-1)) + (grouping(c2) << (n-2)) + … + grouping(cn).",
     grouping_udaf
 );
 
 #[user_doc(
     doc_section(label = "General Functions"),
-    description = "Returns 1 if the data is aggregated across the specified column, or 0 if it is not aggregated in the result set.",
+    description = "Returns the level of grouping, equals to (grouping(c1) << (n-1)) + (grouping(c2) << (n-2)) + … + grouping(cn).",
     syntax_example = "grouping(expression)",
     sql_example = r#"```sql
 > SELECT column_name, GROUPING(column_name) AS group_column
   FROM table_name
   GROUP BY GROUPING SETS ((column_name), ());
-+-------------+-------------+
++-------------+--------------+
 | column_name | group_column |
-+-------------+-------------+
-| value1      | 0           |
-| value2      | 0           |
-| NULL        | 1           |
-+-------------+-------------+
++-------------+--------------+
+| value1      | 0            |
+| value2      | 0            |
+| NULL        | 1            |
++-------------+--------------+
 ```"#,
     argument(
         name = "expression",