> to give you a real world example, we use mariadb for storing everything persistently. however, a lot of data like "number of teachers in school A" is aggregated and too difficult to run in real time when we render paged results. to get around that, we use mongo as a document store and use its SQL like querying to generate the paged search results. this lets us sort/filter on the data without having to do everything in SQL.

This use case should be possible to solve with the JSON type in PostgreSQL. The indexing in PostgreSQL is just as advanced in 9.3 and will be better than MongoDB in 9.4 if a couple of patches land.