If you're the only user and just want it working without much fuss, use a single db instance and forget about it. Less to maintain leads to better maintenance, if performance isn't more important.

It's fairly straightforward to migrate a db to a new postgres instance, so you're not shooting yourself in a future foot if you change your mind.

Use PGTune to get as much as you can out of it and adjust if circumstances change.

There are pros and cons to each...

There's the question of isolation. One shared service going down brings down multiple applications. But it can be more resource efficient to have a single DB running. If you're doing database maintenance it can also be easier to have one instance to tweak. But if you break something it impacts more things...

Generally speaking I lean towards one db per application. Often in separate containers. Isolation is more important for what I want.

I don't think anyone would say you're wrong for using a single shared instance though. It's a more "traditional" way of treating databases actually (from when hardware was more expensive).

Would it be better to just have one PostgreSQL service running that serves both Nextcloud and Lemmy

Yes, performance and maintenance-wise.

If you're concerned about database maintenance (can't remember the last time I had to do this... Once every few years to migrate postgres clusters to the next major version?) bringing down multiple services, setup master-slave replication and be done with it

I have had one database container for each service, because "if something happens"...

Now I have one container for all databases for postgres, finetuned with PGTune, never regretted it. Important is proper backup, like datadump with pg_dumpall

More performance, less overhead. If you are confident enough and experienced in docker, then use postgres on your host instead of a container. But one container for all databases is okay too.

Some people will disagree and this is fine, but this is my way to manage it. It is not this complicated ☺️

Native, docker, something else? How starved are you at the moment?

Here's the docker stats of my Nextcloud containers (5 users, ~200GB data and a bunch of apps installed):

No DB wiz by a long shot, but my guess is that most of that 125MB is actual data. Other Postgres containers for smaller apps run 30-40MB. Plus the container separation makes it so much easier to stick to a good backup strategy. Wouldn't want to do it differently.

Postgres doesn't need that much ram IMO, though it may use as much as you give it. I'd reduce it's ram and see how performance changes.

i like to have them separated by service, i have 3/4 dbs running, at least 2 of them are postgreSQL.

If you're starved for RAM, there's nothing wrong with a shared instance, as long as you're aware of the risk of that single instance bringing down multiple services.

I run a three node Proxmox cluster, and two nodes have 80GB RAM each, so my situation is very different to yours. So, I have four Postgres instances:

1. Mission critical: pretty much my RADIUS database, for wireless auth and not much else (yet)
2. Important: paperless-ngx, and other similarly important services
3. Immich: because Immich has a very specific set of Postgres requirements
4. Meh: 2 x Sonarr, 3 x Radarr, 1 x Lidarr (not fussed if this instances goes down and takes all of those services with it)

Seperate DB container for each service. Three main reasons: 1) if one service requires special configuration that affects the whole DB container, it won’t cross over to the other service which uses that DB container and potentially cause issues, 2) you can keep the version of one of the DB containers back if there is an incompatibility with a newer version of the DB and one of the services that rely on it, 3) you can rollback the dataset for the DB container in the event of a screwup or bad service (e.g. Lemmy) update without affecting other services. In general, I’d recommend only sharing a DB container if you have special DB tuning in place or if the services which use that DB container are interdependent.