Add some clarifications to README.md in the database schema directory

[rubo77]

Please someone explain, what the two folders 16 and 54 are about and why the dump is used for later, and why it is important.

I plan to add some information, how to add the foreign key CONSTRAINT like visualized in #934 like

ALTER TABLE event_forward_extremities ADD CONSTRAINT room_id_fk FOREIGN KEY (room_id) REFERENCES rooms (room_id);
ALTER TABLE room_account_data         ADD CONSTRAINT room_id_fk FOREIGN KEY (room_id) REFERENCES rooms (room_id);
ALTER TABLE room_account_data         ADD CONSTRAINT user_id_fk FOREIGN KEY (user_id) REFERENCES users (name);
ALTER TABLE room_memberships          ADD CONSTRAINT room_id_fk FOREIGN KEY (room_id) REFERENCES rooms (room_id);
ALTER TABLE room_memberships          ADD CONSTRAINT user_id_fk FOREIGN KEY (user_id) REFERENCES users (name);
ALTER TABLE room_stats_state          ADD CONSTRAINT room_id_fk FOREIGN KEY (room_id) REFERENCES rooms (room_id);
ALTER TABLE state_groups              ADD CONSTRAINT room_id_fk FOREIGN KEY (room_id) REFERENCES rooms (room_id);

Flooring keys would be a great way to maintain data integrity, especially as a safeguard against orphaned records.

Maybe we can cleanup script that removes all orphaned entries and then adds all foreign keys to the PostgreSQL Database, or is there already such a script out in the wild somewhere?

[richvdh]

I plan to add some information, how to add the foreign key CONSTRAINT

please don't do this. It will lead to people having different database schemas to what we test and develop against, which will be a support nightmare.

I'm not totally against having foreign-key constraints in the database, but if we do it, we should do it for everyone via delta files, not by manual hackery. In any case, please come and discuss why we should do this in #synapse-dev first, once people are back from the holiday period.

[rubo77]

I think the readme can be merged now

[richvdh]

@rubo77 I appreciate the contribution, and please don't take this the wrong way, but you might consider leaving documentation updates to people whose first language is English. I'm not sure that this is actually adding much, and I feel like I've had to rewrite most of your contributions anyway?

[richvdh]

Suggested change

	for historical reference, and one folder with most recent database-schema.
	for historical reference. `54` is currently the most recent full schema snapshot.

[rubo77]

I changed it to "The other contains the most recent full
schema snapshot." so we wouldn't have to change the Readme every time, we update the schema

[richvdh]

but this will be wrong next time we create a full schema... 🤷‍♂️

[rubo77]

Why is this wron the next time? "the other" is always the latest, isn't it?

[richvdh]

yes, but then there will be three folders.

[richvdh]

the point is: we're going to have to update this paragraph next time we create a full schema anyway, so we may as well be clear which folders we are talking about.