>>it can be an effective custom database engine for any table with less than 4 >>billion rows. >What is the source of the magic number 4 Billion ? The maximum value of Count and Positive_Count in direct_io are implementation defined. On two compilers that I have worked with they were equivalent to long_integer which was defined as 32 bit. If an implementation defined them and long_integer as 64 bit, then I guess then 4 billion ** 2 rows would be easy to work with.