SQL> recreate table t(x int, y int); commit;
SQL> insert into t select rand()*5, rand()*5 from rdb$types, rdb$types; commit;

----------------------------------------------------
-- ascend-индекс по одному полю:
SQL> create index t_x_asc on t(x); commit;
SQL> set planonly;

SQL> select x,max(y) from t group by x;
PLAN (T ORDER T_X_ASC) -- Ok

SQL> select x,min(y) from t group by x;
PLAN (T ORDER T_X_ASC) -- Ok

SQL> set planonly;
SQL> drop index t_x_asc; commit;
----------------------------------------------------

-- descend-индекс по одному полю:
SQL> create DESCENDING index t_x_desc on t(x); commit;
SQL> set planonly;

SQL> select x,max(y) from t group by x;
PLAN SORT ((T NATURAL)) -- ???

SQL> select x,min(y) from t group by x;
PLAN SORT ((T NATURAL)) -- ???

SQL> set planonly;
SQL> drop index t_x_desc; commit;
------------------------------------------------------

-- ascend-индекс по двум полям:
SQL> create index t_x_y_asc on t(x, y); commit;
SQL> set planonly;
SQL> select x,max(y) from t group by x;
PLAN (T ORDER T_X_Y_ASC) -- Ok

SQL> select x,min(y) from t group by x;
PLAN (T ORDER T_X_Y_ASC) -- Ok

SQL> set planonly;
SQL> drop index t_x_y_asc;
------------------------------------------------------

-- descend-индекс по двум полям:
SQL> create DESCENDING index t_x_y_desc on t(x, y); commit;
SQL> set planonly;

SQL> select x,max(y) from t group by x;
PLAN SORT ((T NATURAL)) -- ???

SQL> select x,min(y) from t group by x;
PLAN SORT ((T NATURAL)) -- ???

SQL> set planonly;
SQL> drop index t_x_y_desc; commit;