Group By Hint in the Optimized Hint category

/*+USE_HASH_AGGREGATION([qb_name]) */

-- Use the USE_HASH_AGGREGATION hint to force the query optimizer to use the hash aggregation algorithm for aggregation calculations.
-- In this query, this means that the sum of salaries grouped by e.department_id will be calculated using a hash table.
SELECT /*+ USE_HASH_AGGREGATION */ e.department_id, sum(e.salary)
FROM employees e
WHERE e.department_id = 1001
GROUP BY e.department_id;

/*+ NO_USE_HASH_AGGREGATION([qb_name]) */

-- Use the NO_USE_HASH_AGGREGATION hint to instruct the query optimizer to avoid using the hash aggregation algorithm.
-- The optimizer will choose other aggregation methods, such as sort aggregation, to calculate the sum of salaries grouped by e.department_id.
SELECT /*+ NO_USE_HASH_AGGREGATION */ e.department_id, sum(e.salary)
FROM employees e
WHERE e.department_id = 1001
GROUP BY e.department_id;

/*+ GBY_PUSHDOWN([qb_name]) */

SELECT /*+ GBY_PUSHDOWN */ department_id, COUNT(*) as emp_count
FROM employees
GROUP BY department_id;

/*+ NO_GBY_PUSHDOWN([qb_name]) */

SELECT /*+ NO_GBY_PUSHDOWN */ department_id, COUNT(*) as emp_count
FROM employees
GROUP BY department_id;

hint: disable_op_rich_format(op_list);

opt_list: op_name | op_list,op_name

op_name:
LIMIT
| MERGE_DISTINCT
| HASH_DISTINCT
| MATERIAL
| MERGE_JOIN
| GRANULE_ITERATOR
| EXPAND
| SORT
| HASH_UNION
| HASH_INTERSECT
| HASH_EXCEPT
| MERGE_UNION
| MERGE_INTERSECT
| MERGE_EXCEPT
| TABLE_SCAN
| HASH_JOIN
| NESTED_LOOP_JOIN
| SUBPLAN_FILTER
| SUBPLAN_SCAN
| UNPIVOT
| SCALAR_AGGREGATE
| MERGE_GROUP_BY
| HASH_GROUP_BY
| WINDOW_FUNCTION

SELECT /*+disable_op_rich_format('table_scan','hash_group_by')*/ * FROM t GROUP BY a;