MDEV-30660 Aggregation functions fail to leverage uniqueness property

When executing a statement of the form
  SELECT AGGR_FN(DISTINCT c1, c2,..,cn) FROM t1,
where AGGR_FN is an aggregate function such as COUNT(), AVG() or SUM(),
and a unique index exists on table t1 covering some or all of the
columns (c1, c2,..,cn), the retrieved values are inherently unique.
Consequently, the need for de-duplication imposed by the DISTINCT
clause can be eliminated, leading to optimization of aggregation
operations.
This optimization applies under the following conditions:
  - only one table involved in the join (not counting const tables)
  - some arguments of the aggregate function are fields
        (not functions/subqueries)

This optimization extends to queries of the form
  SELECT AGGR_FN(c1, c2,..,cn) GROUP BY cx,..cy
when a unique index covers some or all of the columns
(c1, c2,..cn, cx,..cy)

sql/sql_select.h

@@ -1905,6 +1905,8 @@ private:
   bool add_fields_for_current_rowid(JOIN_TAB *cur, List<Item> *fields);
   void free_pushdown_handlers(List<TABLE_LIST>& join_list);
   void init_join_cache_and_keyread();
+  bool prepare_sum_aggregators(THD *thd,Item_sum **func_ptr,
+                               bool need_distinct);
 };
 
 enum enum_with_bush_roots { WITH_BUSH_ROOTS, WITHOUT_BUSH_ROOTS};