-- создаём таблицу, вводим три строки, выполняем update одной из них (без commit'a):
C:\1INSTALL\FIREBIRD\Data>isql -n aaa.fdb
Database:  aaa.fdb
SQL> recreate table t(id int, f01 int); commit;
SQL> recreate table t(id int primary key, f01 int); commit;
SQL> create descending index t_idx on t(id); commit;
SQL> insert into t values(1,100);
SQL> insert into t values(2,200);
SQL> insert into t values(3,300);
SQL> commit;
SQL>  update t set f01=199 where id=2; 

C:\1INSTALL\FIREBIRD\Data>isql -n aaa.fdb
Database:  aaa.fdb

---------- проверка доступности  записи  -----------

SQL> commit; set transaction read committed record_version;
SQL> update t set f01=201 where id=2;
^C -- срубил ввиду бесконечного ожидания

C:\1INSTALL\FIREBIRD\Data>isql -n aaa.fdb
Database:  aaa.fdb
SQL> commit; set transaction read committed no record_version;
SQL> update t set f01=201 where id=2;
^C -- срубил ввиду бесконечного ожидания

C:\1INSTALL\FIREBIRD\Data>isql -n aaa.fdb
Database:  aaa.fdb
SQL> commit; set transaction read committed record_version lock timeout 4;
SQL> update t set f01=201 where id=2;
Statement failed, SQLSTATE = 40001
lock time-out on wait transaction
-deadlock
-update conflicts with concurrent update
-concurrent transaction number is 30

SQL> commit; set transaction read committed no record_version lock timeout 4;
SQL> update t set f01=201 where id=2;
Statement failed, SQLSTATE = 40001
lock time-out on wait transaction
-deadlock

SQL> commit; set transaction snapshot lock timeout 4;
SQL> update t set f01=201 where id=2;
Statement failed, SQLSTATE = 40001
lock time-out on wait transaction
-deadlock
-update conflicts with concurrent update
-concurrent transaction number is 30

---------- проверка доступности  чтения  -----------

SQL> commit; set transaction read committed record_version lock timeout 4;
SQL> select * from t;

          ID          F01
============ ============
           1          100
           2          200
           3          300

SQL> commit; set transaction read committed  no  record_version lock timeout 4;
SQL> select * from t;

          ID          F01
============ ============
           1          100
Statement failed, SQLSTATE = 40001
lock time-out on wait transaction
-deadlock

SQL> commit; set transaction snapshot lock timeout 4;
SQL> select * from t;

          ID          F01
============ ============
           1          100
           2          200
           3          300

-- пробуем при NO record_version прочитать всё, за исключением "проблемной" записи:
SQL> commit; set transaction read committed  no  record_version lock timeout 4;
SQL> select * from t where  id<>2 ;

          ID          F01
============ ============
           1          100
Statement failed, SQLSTATE = 40001
lock time-out on wait transaction
-deadlock

-- то же самое, но используя навигацию индексу:
SQL> set plan on;
SQL> commit; set transaction read committed  no  record_version lock timeout 4;
SQL> select * from t where id<>2 order by id desc;

PLAN (T ORDER T_IDX)

          ID          F01
============ ============
           3          300
Statement failed, SQLSTATE = 40001
lock time-out on wait transaction
-deadlock