[forgetfulness]

Should it be used as a database in the way that Postgres would be used as a database? It seems pretty unfit for that role, with its past durability issues and that writes have to be tailored to how the data will be consumed, due to lack of ad-hoc queries.

As something layered on top of the actual source of truth it may be reasonable, writing a materialized version of a costly query that's read very often, sure, but that's an optimization and it'd be competing with Redis for what matters there.

[afiori]

MongoDB has pipeline queries https://www.mongodb.com/docs/manual/core/aggregation-pipelin... that allow for quite ad-hoc queries.

[deepsun]

Same thoughts. I see mongo as a key-value store (string key -> string value), with a json wrapper over the values.