--drop constraint se existe
DECLARE
itemExists NUMBER;
BEGIN
     itemExists := 0;

SELECT COUNT(CONSTRAINT_NAME) INTO itemExists 
FROM ALL_CONSTRAINTS
WHERE UPPER(CONSTRAINT_NAME) = UPPER('TARIFA_UNICA');

IF itemExists > 0 THEN
EXECUTE IMMEDIATE 'ALTER TABLE TARIFA DROP CONSTRAINT TARIFA_UNICA';
END IF;

END;

-- drop index se existe
DECLARE
  COUNT_INDEXES INTEGER;
BEGIN
  SELECT COUNT(*) INTO COUNT_INDEXES
    FROM USER_INDEXES
    WHERE INDEX_NAME = 'TARIFA_UNICA';

  IF COUNT_INDEXES > 0 THEN
    EXECUTE IMMEDIATE 'DROP INDEX TARIFA_UNICA';
  END IF;
END;