So, this is the first project where we're gonna use postgres in Kolab?

It is not clear that kolab-chat will have any need for an SQL database. Currently, starting Ecto in lib/kolab_chat.ex is commented out since it is not currently using any database at the moment ... and I am not even sure we will need to or want to. The mix ecto steps can probably be skipped entirely during project build right now!

It looks like storage requirements for kolab-chat may be limited to room configurations and logging (room histories, etc), and an SQL database may not be the best solution for that. As we spec out more of the application's features and components, this will become more clear.

So I'm not concerned about making any decisions right now, as we may not have to make a decision at all :)

Maybe we should use mysql by default?

You can change this locally in config/dev.exs during development, so you don't have to have or set up a pgsql instance on your machine.

If kolab-chat does end up requiring an SQL database, then we should discuss this indeed, though in the scope of Kolab as a whole. Currently the default for deployment is MySQL (and kolab-chat can easily use that via a config file change), but long term that is something that needs to be discussed and determined further.