session #1 
C:\MIX\firebird\fb25>isql T0.FDB
Database:  T0.FDB
SQL> create table t(id int, f01 int); commit;
SQL> set term ^;
SQL> execute block as
CON> declare i int=2;
CON> begin
CON> while (i<=1000000) do insert into t values(:i, :i/2) returning :i+1 into i;
CON> end^
SQL> set term ;^
SQL> commit;
SQL> select first 5 rdb$db_key,id,f01 from t;

DB_KEY                     ID          F01
================ ============ ============
8000000001000000            2            1
8000000002000000            3            1
8000000003000000            4            2
8000000004000000            5            2
8000000005000000            6            3

SQL> select first 5 rdb$db_key,id,f01 from t where id>=999995;

DB_KEY                     ID          F01
================ ============ ============
800000009DA03000       999995       499997
800000009EA03000       999996       499998
800000009FA03000       999997       499998
80000000A0A03000       999998       499999
80000000A1A03000       999999       499999
-- как видно, младшим значениям поля ID соотв-вуют меньшие значения RDB$DB_KEY, т.е. записи легли в натуральном порядке.
-- Теперь удаляем первые 10 строк, делаем commit:
SQL> delete from t where id<=10; commit;

 session #2 
C:\MIX\firebird\fb25>isql T0.FDB
Database:  T0.FDB
SQL> insert into t values(-1,  1000 ); commit; -- Enter пока НЕ нажимаем!

 session #1 
SQL> set transaction read committed record_version no wait;
SQL> update t set f01= null  where id>-999999; -- и жамкаем ENTER

 session #2 
[Enter]
SQL> _ -- выполнилось ОК.

 session #1 
SQL> select rdb$db_key,id,f01 from t where id>=999995;

DB_KEY                     ID          F01
================ ============ ============
800000009DA03000       999995       <null>
800000009EA03000       999996       <null>
800000009FA03000       999997       <null>
80000000A0A03000       999998       <null>
80000000A1A03000       999999       <null>
80000000A2A03000      1000000       <null>

SQL> select rdb$db_key,id,f01 from t where id=-1;

DB_KEY                     ID          F01
================ ============ ============
80000000 A7E83000            -1        <null> 

 session #1 
C:\MIX\firebird\fb25>isql
Use CONNECT or CREATE DATABASE to specify a database
SQL> create database 't0.fdb'; commit;
SQL> create table t(id int, f01 int); commit;
SQL> set term ^;
SQL> execute block as
CON> declare i int=2;
CON> begin
CON> while (i<=2000000) do insert into t values(:i, :i/2) returning :i+1 into i;
CON> end^
SQL> set term ;^
SQL> commit;
SQL> select first 5 rdb$db_key,id,f01 from t;

DB_KEY                     ID          F01
================ ============ ============
8000000001000000            2            1
8000000002000000            3            1
8000000003000000            4            2
8000000004000000            5            2
8000000005000000            6            3

SQL> delete from t where id<=1000000; commit;
SQL> select count(*) from t; commit;

       COUNT
============
      1000000 

 session #2 
C:\MIX\firebird\fb25>isql T0.FDB
Database:  T0.FDB
SQL> insert into t values(-1,  1000 ); commit; -- Enter пока НЕ жмём!

 session #1 
SQL> set transaction  read committe d record_version no wait;
SQL> update t set f01= null  where id > -999999;

 session #2  (быстро перескакиваем сюда по Alt-Tab):
[Enter]
SQL> _

 session #1 
SQL> select rdb$db_key,id,f01 from t where id < 0;

DB_KEY                     ID          F01
================ ============ ============
8000000076370000           -1          1000 

update t set f01=null where id > -999999

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PLAN (T NATURAL)

0 records fetched

  16617 ms, 13337 read(s), 12782 write(s), 9309604 fetch(es), 2136164 mark(s)



Table                             Natural     Index    Update    Insert    Delete   Backout     Purge   Expunge

***************************************************************************************************************

T                                 1000000             1000000                                                  

SQL> select rdb$db_key,id,f01 from t where id >= 1999995;

DB_KEY                     ID          F01
================ ============ ============
8000000051406100      1999995       <null>
8000000052406100      1999996       <null>
8000000053406100      1999997       <null>
8000000054406100      1999998       <null>
8000000055406100      1999999       <null>
8000000056406100      2000000       <null>