I always heard it as "if the database index fits in the RAM of a single machine, it's not big data". The reason being that this makes random access fast. You al

I always heard it as "if the database index fits in the RAM of a single machine, it's not big data". The reason being that this makes random access fast. You always know where a piece of data is. Once the index is too big to have in one place, thing get more complicated.