create table T (
    ID   integer not null,
    INC  integer
);

alter table T
  add constraint PK_T primary key ( ID );

insert into T ( ID )
       values ( 1 );
insert into T ( ID )
       values ( 2 );
insert into T ( ID )
       values ( 3 );
commit;

ID	INC
1	null
2	null
3	null

set term ^ ;
create procedure P_INC
returns ( INC integer )
as
begin
-- процедура в рамках транзакции возвращает монтонно возрастающую последовательность
  RDB$SET_CONTEXT( 'USER_TRANSACTION', 'INC', cast( coalesce( RDB$GET_CONTEXT( 'USER_TRANSACTION', 'INC'), 0 ) as integer ) + 1 );
  INC = RDB$GET_CONTEXT( 'USER_TRANSACTION', 'INC' );
  suspend;
end ^

set term ; ^

merge into T as a
using ( select ID,
               p.INC as INC
          from T
          left join PR_INC as p on ( 1 = 1 )
         order by ID ) as b
on ( a.ID = b.ID )
when matched then
  update
     set INC = b.INC;

ID	INC
1	1
2	2
3	3

merge into T as a
using ( select ID,
             ( select INC from PR_INC ) as INC
          from T
         order by ID ) as b
on ( a.ID = b.ID )
when matched then
  update
     set INC = b.INC;

ID	INC
1	1
2	1
3	1

select ID,
     ( select INC from PR_INC ) as INC
  from T
 order by ID

ID	INC
1	1
2	2
3	3

create or alter procedure PR_INC ( ID integer )
returns ( INC integer)
as
begin
-- входной параметр ID нигде не используется
  RDB$SET_CONTEXT( 'USER_TRANSACTION', 'INC', cast( coalesce( RDB$GET_CONTEXT( 'USER_TRANSACTION', 'INC'), 0 ) as integer ) + 1 );
  INC = RDB$GET_CONTEXT( 'USER_TRANSACTION', 'INC' );
  suspend;
end

merge into T as a
using ( select ID,
             ( select INC from PR_INC( t.ID ) ) as INC
          from T
         order by ID ) as b
on ( a.ID = b.ID )
when matched then
  update
     set INC = b.INC;