SELECT 0x8000000000000000 FROM RDB$DATABASE; --int64

             CONSTANT 
===================== 
 -9223372036854775808 

SELECT 0x18000000000000000 FROM RDB$DATABASE; --int128

             CONSTANT 
===================== 
Statement failed, SQLSTATE = 22003
Dynamic SQL Error
-SQL error code = -303
-arithmetic exception, numeric overflow, or string truncation
-Integer overflow.  The result of an integer operation caused the most significant bit of the result to carry.