The approach here is very different. It's not that window functions are bad, but that the traditional SQL database, or even column oriented database, doesn't have a disk format that is versed to this type of analysis for the high volumes that AdRoll has. TrailDB can filter through 10s of millions of events per second on a Macbook, just storing billions of events in a more traditional DB or warehouse would use far more than the space that TrailDBs use. On top of that it's relatively easy to build visualisations on top of these databases with an interactive UI to do exploration. So the window function being complicated was perhaps just the spark but there are real advantages in speed, efficiency and expressiveness in this type of database.