/*+ NDEX_FOR_GROUPBY ([@query_block_name] tbl_name [index_name])*/

CREATE TABLE t1 (
  a INT,
  b INT,
  c INT,
  KEY k2 (a, b, c)
);

INSERT INTO t1 VALUES
(1, 2, 3), (1, 2, 4), (3, 4, 5), (3, 4, 5);

SELECT /*+ INDEX_FOR_GROUPBY(t1 k2)*/DISTINCT a, b
  FROM t1
 WHERE a BETWEEN 1 AND 10
   AND a > 1 AND c = 5 
ORDER BY a, b;