[lmm]

I meant just in terms of Postgres expecting a "standard" relational schema; the interface and query planner and the like are designed around that. It's not a bad schemaless data store per se, but it's not really designed for that use case, so I would expect rough edges when using it that way.

The modern selection of schemaless data stores weren't as mature (if they even existed) when Reddit was getting started, so don't assume that they would choose that architecture if they were starting today. I suspect that if you build on postgres in this way you'll have to write significant pieces of functionality by hand that you get for free with mongodb/couchdb/redis/neo4j/etc.

That said I'll emphasise again that I don't know too much about your system; at some point, prototyping and measuring will be the only way to see what works best for you.