recreate table test(id int);
commit;
insert into test(id)
with recursive r as(select 0 i from rdb$database union all select r.i+1 from r where r.i<17)
select r1.i*18 + r0.i
from r r1, r r0
 rows 300 ;
commit;

create index test_id on test(id);
commit;
set list on;

select rdb$statistics
from rdb$indices
where rdb$index_name = upper('test_id');

select 1.000000000000000000/nullif(count(distinct(id)), 0)
from test;

RDB$STATISTICS                  0.003333333414047957
DIVIDE                          0.003333333333333333

SQL> recreate table test(idx_stat double precision);
SQL> commit;
SQL> insert into test select 1.000000000000000000/300 from rdb$database returning idx_stat;

               IDX_STAT
=======================
   0.003333333333333333

SQL> select cast(1.000000000000000/300 as float) from rdb$database;

          CAST
==============
  0.0033333334

SQL> select cast(cast(1.000000000000000/300 as float) as double precision) from rdb$database;

                   CAST
=======================
   0.0033333334 14047957