Most of the existing answers here completely miss the point of your question. PostgreSQL now has great native key/value and JSON support, so if you want to go "schemaless" likeand leave modelling your domain until you already have a heap of data, you can just use an HStore/JSON column and route data straight to it. You can still use schemas on other tables without incurring the operational headache of using two different databases.High availability is not the same as write throughput, in fact they are somewhat opposed. Super-high "write" throughputs can be obtained in Mongo by configuring it to defer actually writing to disk until later. This makes it possible for you to lose data, the opposite of availability. PostgreSQL has no such option, so may be inappropriate if you don't want your data actually persisted.As of version 9.4, PostgreSQL benchmarks faster than MongoDB for both inserting and querying JSON data. See(though no doubt both databases could be tuned for the specific benchmark).See also some good answers toandIn summary: probably never for any technical reasons.