execute block returns (empty_table varchar(31)) as
declare v_cnt smallint;
declare v_sql varchar(255);
begin
    for
        select r.rdb$relation_name
        from rdb$relations r
        where coalesce(r.rdb$system_flag,0)=0 -- не смотрим на системные таблицы
        and r.rdb$relation_type not in(4,5) -- убираем GTT'шки
        and r.rdb$view_source is null -- убираем вьюхи
    into empty_table
    do begin
      v_sql='select count(*) from (select first 1 * from '||empty_table||')';
      execute statement (v_sql) into :v_cnt;
      if (v_cnt=0) then suspend;
    end
end

Execute time = 32ms
Avg fetch time = 2,46 ms
Current memory = 19 143 936
Max memory = 19 351 896
Memory buffers = 1 024
Reads from disk to cache = 0
Writes from cache to disk = 0
Fetches from cache = 1 327

select first 1  1  from

C:\1INSTALL\FIREBIRD\FB_25>isql 192.168.0.60:/var/db/firebird/somebase.fdb -user sysdba -pas masterke
Database:  192.168.0.60:/var/db/firebird/somebase.fdb, User: sysdba
SQL> set term ^;
SQL> execute block returns (empty_table varchar(31)) as
CON> declare v_cnt smallint;
CON> declare v_sql varchar(255);
CON> begin
CON>     for
CON>         select r.rdb$relation_name
CON>         from rdb$relations r
CON>         where coalesce(r.rdb$system_flag,0)=0 -- не смотрим на системные таблицы
CON>         and r.rdb$relation_type not in(4,5) -- убираем GTT'шки
CON>         and r.rdb$view_source is null -- убираем вьюхи
CON>     into empty_table
CON>     do begin
CON>       v_sql='select 1  1  from '||empty_table||'';
CON>       execute statement (v_sql) into :v_cnt;
CON>       if (v_cnt is null) then suspend;
CON>     end
CON> end^

EMPTY_TABLE
===============================
Statement failed, SQLSTATE = 42000
Dynamic SQL Error
-SQL error code = -104
-Token unknown - line 1, column 10
-1
SQL> set term ;^
SQL> commit;

SQL> select first 1 1 from doc;

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

execute block returns (empty_table varchar(31)) as
declare v_cnt smallint;
declare v_sql varchar(255);
begin
    for
        select r.rdb$relation_name
        from rdb$relations r
        where coalesce(r.rdb$system_flag,0)=0 -- не смотрим на системные таблицы
        and r.rdb$relation_type not in(4,5) -- убираем GTT'шки
        and r.rdb$view_source is null -- убираем вьюхи
    into empty_table
    do begin
      v_cnt=null;
      v_sql='select first 1 1 from '||empty_table||'';
      execute statement (v_sql) into :v_cnt;
      if (v_cnt is null) then suspend;
    end
end