create table t1 (id bigint not null, primary key(id));
create table t2 (id bigint not null, primary key(id));
create table t3 (id bigint GENERATED BY DEFAULT AS IDENTITY not null, id_t1 bigint not null, id_t2 bigint not null, flg1 smallint default 0, flg2 smallint default 0,
                 primary key(id), foreign key (id_t1) REFERENCES t1(id), foreign key (id_t2) REFERENCES t2(id));
create index t3_idx on t3 (id_t2,flg1);
commit work;

set term ^;
execute block
as
declare variable i int=1;
begin
insert into t2(id)
select 1 from rdb$database
union
select 2 from rdb$database;

while (:i<=2000000)
do
begin
insert into t1(id)
values(:i);

insert into t3(id_t1,id_t2)
values(:i, 1);

insert into t3(id_t1,id_t2)
values(:i, 2);

i=:i+1;
end
end^

set term ;^

update t3 set flg1=1 where id<=500000;

commit work;

set statistics index t3_idx;

commit work;

Query
------------------------------------------------
execute block
as
declare variable ID bigint;
begin
  for select ID
      from T3
      where ID_T2 = 1 and
            FLG1 = 0 and
            FLG2 = 0
      into :ID
  do
  begin
    break;
  end
end

Plan
------------------------------------------------
PLAN (T3 INDEX (T3_IDX))

Query Time
------------------------------------------------
Prepare       : 0,00 ms
Execute       : 156,00 ms
Avg fetch time: 0,00 ms

Memory
------------------------------------------------
Current: 1 120 421 232
Max    : 1 157 990 336
Buffers: 65 536

Operations
------------------------------------------------
Read   : 0
Writes : 0
Fetches: 253 075
Marks  : 0


Enchanced Info:
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
|          Table Name           |  Records  |  Indexed  | Non-Indexed | Updates | Deletes | Inserts | Backouts |  Purges  | Expunges |
|                               |   Total   |   reads   |    reads    |         |         |         |          |          |          |
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
|T3                             |         0 |    250001 |           0 |       0 |       0 |       0 |        0 |        0 |        0 |
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+

Query
------------------------------------------------
execute block
as
declare variable ID bigint;
begin
  for select ID
      from T3
      where ID_T2 = 1 and
            FLG1 = 0 and
            FLG2 = 1
      into :ID
  do
  begin
    break;
  end
end

Plan
------------------------------------------------
PLAN (T3 INDEX (T3_IDX))

Query Time
------------------------------------------------
Prepare       : 0,00 ms
Execute       : 1 139,00 ms
Avg fetch time: 0,00 ms

Memory
------------------------------------------------
Current: 1 120 421 232
Max    : 1 157 990 336
Buffers: 65 536

Operations
------------------------------------------------
Read   : 0
Writes : 0
Fetches: 2 016 331
Marks  : 0


Enchanced Info:
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
|          Table Name           |  Records  |  Indexed  | Non-Indexed | Updates | Deletes | Inserts | Backouts |  Purges  | Expunges |
|                               |   Total   |   reads   |    reads    |         |         |         |          |          |          |
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+
|T3                             |         0 |   2000000 |           0 |       0 |       0 |       0 |        0 |        0 |        0 |
+-------------------------------+-----------+-----------+-------------+---------+---------+---------+----------+----------+----------+