text: DROP TABLE IF EXISTS t;
  CREATE TABLE t
    (
       c_a INT,
       c_b INT,
       c_c INT
    );
  INSERT INTO t
  VALUES  (1,1,1),
          (2,2,2),
          (3,3,3);
  CREATE UNIQUE INDEX idx
    ON t(c_a, c_b);
  CREATE INDEX idx_a
    ON t(c_a, c_c);
  --exists select list
  SELECT /*+ recompile */ count(b.c_a)
  FROM   t a
         LEFT OUTER JOIN t b
                      ON a.c_a = b.c_a
                     AND a.c_b = b.c_b
  WHERE  a.c_c = 1; 
  SELECT /*+ recompile */ count(a.c_a)
  FROM   t a
         LEFT OUTER JOIN t b
                      ON a.c_a = b.c_a
                     AND a.c_b = b.c_b
  WHERE  a.c_c = 1; 
  DROP TABLE t;
  