The answer to this question would be extremely long, so I suggest you first brush up on some fundamentals of database design.

http://www.slideshare.net/bijugs/row-or-columnar-based-datab...

then go onto getting mad at Michael Stonebraker here:

https://gigaom.com/2013/12/14/5-predictions-on-the-future-of...

and then when you realize he's right about everything, you'll have the foundational knowledge to understand the problem a little bit better.