SSH to mattermost.fysiksektionen.se as root
Connect to database
su - postgres
psql
\c mattermost
Then normal SQL querys, strings must be in '.
Undelete one message:
UPDATE posts SET deleteat = 0 WHERE id='chowcd1f7bni9xs6mnc1ttwgny'
Undelete multiple messages:
UPDATE posts SET deleteat = 0
WHERE id IN ('chowcd1f7bni9xs6mnc1ttwgny',...);
Be Careful! This writes directly to PROD!!! Do not undelete every message on the server for example :)
\l - lists databases
\c [database] - connects to database
\dt - lists tables
\d+ [table] - shows more information on specific table
Here is a postgres sheet-cheat if needed.