There was also (not sure if still developed) an interface called ToroDB, which allows you to plug in a MongoDB application to store data in Postgres. The interface provides Mongo compatible layer and the application still thinks it talks with Mongo. Even with the extra translation layer, ToroDB was faster than original MongoDB[2].

[1] https://www.enterprisedb.com/postgres-plus-edb-blog/marc-lin...

[2] https://www.slideshare.net/8kdata/torodb-internals-how-to-cr...