Cluster is very easy in MongoDB and for unstructured data. I've looked into clustering postgresql a few time and it seems like a pain in the butt. TBH, storing data without thinking about relation is just kicking the can down the road.

My previous company store coupon code on it (referrals). I prefer a hash nosql for that though (cassandra) but for a one man shop it's overkill.

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IIRC postgresql have their own document storage like MongoDB. I just never had to use such thing because I guess I'm too OCD about how my data suppose to look like.