One of the challenges of big data is that it is, well, big.
Computers are optimized for math on 64 bits or less. Any bigger,
and extra steps have to be taken to work with the data which is
very expensive. This is why a BIGINT is 64 bits. In MySQL
DECIMAL can store more than 64 bits of data using fixed
precision. Large numbers can use FLOAT or DECIMAL but those
data types are lossy.
DECIMAL is an expensive encoding. Fixed precision math is expensive and you eventually run out of precision at which point you can't store any more data, right?
What happens when you want to store a counter that is bigger than the maximum DECIMAL? FLOAT is …