stampede VS FerretDB

It was both. There were two separate software based on the same underlying technology:

* ToroDB Stampede[1]: MongoDB replica, converting on-the-fly documents to relational structures. Targeting OLAP, as data normalization made queries from some % faster to 2-3 orders of magnitude faster.

* ToroDB Server[2]: what DocumentDB is or FerretDB is planning to be. It was less developed than Stampede, certainly.

[1]: https://github.com/torodb/stampede/

if you have postgres, just use https://github.com/FerretDB/FerretDB

FerretDB is an open source document database that adds MongoDB compatibility to other database backends, such as Postgres and SQLite. By using FerretDB, developers can access familiar MongoDB features and tools using the same syntax and commands for many of their use cases.

There is FerretDB. But they are not fully compatible to Mongo yet.

https://www.ferretdb.io/

Feature parity progress with apps:

https://github.com/FerretDB/FerretDB/issues/5

I presume, that after there is feature parity, then someone could run some benchmarks.

Yep that's serious issue and it's similar to our case. Our main product can work just fine even without SSE 4.2 but MongoDB requires it and then indirectly leads to AVX1 support as we use MongoDB as storage. We did PoC with FerretDB last month and I think it may be good option for Gerylog: https://www.ferretdb.io

Shout out to FerretDB, which is an in progress open source re-implementation of MongoDB atop PostgreSQL.

https://github.com/FerretDB/FerretDB