recreate table tx(s varchar(1000)); commit;
insert into tx select 'a' from rdb$types rows 200;
commit;
set term ^;
execute block as
declare n int = 500000;
begin
  while(n>0) do
    insert into tx values(lpad(uuid_to_char(gen_uuid()),1000,'b'))
    returning :n-1 into n;
end
^set term ;^
commit;
insert into tx select 'c' from rdb$types rows 200;
commit;
create index tx_s on tx(s);
commit;

select s from (select s from tx order by s) where s >= :s order by s rows 1

PLAN  SORT  (TX ORDER TX_S)

Execute time = 4s 343ms
Avg fetch time = 4 343,00 ms
Current memory = 0
Max memory = 0
Memory buffers = 524 288
Reads from disk to cache = 0
Writes from cache to disk = 0
Fetches from cache = 1 502 565

select s from (select s from tx) where s >= :s order by s rows 1

SQL> set stat on;
SQL> set explain on;

-- хороший индеец:
SQL> select 1 from (select s from tx) where s >= 'a' order by s rows 1;

Select Expression
    -> First N Records
        -> Filter
            -> Table "TX" Access By ID
                -> Index "TX_S" Range Scan (lower bound: 1/1)

    CONSTANT
============
           1

Current memory = 8898785576
Delta memory = 419112
Max memory = 9539762224
Elapsed time= 0.002 sec
Cpu = 0.000 sec
Buffers = 524288
Reads = 0
Writes = 0
Fetches = 98


-- плохой индеец (BTW: зачем тут два раза "Filter" ?):
SQL> select 1 from (select s from tx order by s) where s >= 'a' order by s rows 1;

Select Expression
    -> First N Records
        -> Sort (record length: 1026, key length: 1008)
            -> Filter
                -> Filter
                    -> Table "TX" Access By ID
                        -> Index "TX_S" Range Scan (lower bound: 1/1)

    CONSTANT
============
           1

Current memory = 8899050816
Delta memory = 265240
Max memory = 9541833272
Elapsed time= 4.232 sec
Cpu = 0.000 sec
Buffers = 524288
Reads = 0
Writes = 0
Fetches = 1502569