>>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.