Stitch Fix seems to be one of those online services that send you sets of clothes that they think fit your style. That seems like a really narrow, low-data kind of industry. How much data can they possibly have? And why so big on the backend? In January 2018 they had 100 engineers. Presumably they're even larger now. Just for a service that sends out clothing.
Maybe I'm lacking in imagination or insight into what takes to run a company like this. On the other hand, a single PostgreSQL instance can run complex ad-hoc queries, with CTEs and everything, on a single node involving millions, even billions, of rows.
They also have mobile apps, they run product experiments, they source and sell clothing and manage inventory, they build and iterate on algorithmic approaches to recommend and design clothing (many of which help stylists and never reach the screen of an external client).
You can skim through their Algorithms blog for some more detail. I find them impressive in how they scale the impact of relatively few stylists to about 3M users.
It is a distributed compute engine that has a lot of capabilities, is rock solid, allows you to blend SQL with Python/R/Scala and can support ML use cases as your needs grow. You can easily store all of your data in PostgreSQL and run Spark on top.