Out of curiosity, did you guys consider running SQL on MongoDB using Postgres foreign tables? What were the pros and cons of that approach for your use-cases?
(In full disclosure, I wrote mongo_fdw for PostgreSQL.)
I actually prototyped our PostgreSQL solution using mongo_fdw (Incidentally, I throw together Debian packaging here, if you're interested: https://github.com/nelhage/mongo_fdw).
Our experience was that mongo_fdw doesn't (yet?) give postgres enough information and knobs to plan JOINs efficiently, which is one of the things we wanted. I got a decent amount of leverage out of using mongo_fdw and then cloning to native tables using SELECT INTO, though :)
That's neat. :) For mongo_fdw, we're expecting better join performance once we upgrade to PostgreSQL 9.2 FDW APIs and start collecting table statistics.
I'm sure there are other factors involved for MoSQL, but they are probably outside the scope of this post. I'd love chat about them offline.
Our experience was that mongo_fdw doesn't (yet?) give postgres enough information and knobs to plan JOINs efficiently, which is one of the things we wanted. I got a decent amount of leverage out of using mongo_fdw and then cloning to native tables using SELECT INTO, though :)