SQL> select cast( 1 as decimal(18,8) ) * cast( 1 as decimal(18,8) ) from rdb$database;

             MULTIPLY
=====================
   1.0000000000000000

SQL> select cast( 1 as decimal(18,9) ) * cast( 1 as decimal(18,9) ) from rdb$database;

             MULTIPLY
=====================
 1.000000000000000000

SQL> select cast( 1 as decimal(18,10) ) * cast( 1 as decimal(18,10) ) from rdb$database;

             MULTIPLY
=====================
Statement failed, SQLSTATE = 22003
Integer overflow.  The result of an integer operation caused the most significant bit of the result to carry.