[kbenson]

Given the time of that question, my guess would be they are using floating point underneath, and have worked out the math so you get approximately that much precision. There's some weirdness in this between 32 bit and 64 bit, based on how the calculation is done. See [1].

I had a mysql instance once where I migrated from a 32 bit system to a 64 bit system, and one of the main tables' primary key was a float that was a high resolution timestamp. Unfortunately, on inserting the old data into the new 64 bit system with a MySQL compiled for 64 bit, the values started having lots of collisions on the primary key, where two previously distinct high-resolution float values now rounded to the same float in 64 bit. Counterintuitive, and a nightmare.

1: https://stackoverflow.com/questions/31415712/floating-point-...