Add domain name change instructions to docs.

docs/src/administration_backup_and_restore.md

@@ -37,6 +37,41 @@ cat db_dump.sql  |  docker exec -i FOLDERNAME_postgres_1 psql -U lemmy # restore
 docker exec -i FOLDERNAME_postgres_1 psql -U lemmy -c "alter user lemmy with password 'bleh'"
 ```
 
+### Changing your domain name
+
+If you haven't federated yet, you can change your domain name in the DB. **Warning: do not do this after you've federated, or it will break federation.**
+
+```
+-- Post
+update post set ap_id = replace (ap_id, 'old_domain', 'new_domain');
+update post set url = replace (url, 'old_domain', 'new_domain');
+update post set body = replace (body, 'old_domain', 'new_domain');
+update post set thumbnail_url = replace (thumbnail_url, 'old_domain', 'new_domain');
+
+delete from post_aggregates_fast;
+insert into post_aggregates_fast select * from post_aggregates_view;
+
+-- Comments
+update comment set ap_id = replace (ap_id, 'old_domain', 'new_domain');
+update comment set content = replace (content, 'old_domain', 'new_domain');
+
+delete from comment_aggregates_fast;
+insert into comment_aggregates_fast select * from comment_aggregates_view;
+
+-- User
+update user_ set actor_id = replace (actor_id, 'old_domain', 'new_domain');
+update user_ set avatar = replace (avatar, 'old_domain', 'new_domain');
+
+delete from user_fast;
+insert into user_fast select * from user_view;
+
+-- Community
+update community set actor_id = replace (actor_id, 'old_domain', 'new_domain');
+
+delete from community_aggregates_fast;
+insert into community_aggregates_fast select * from community_aggregates_view;
+```
+
 ## More resources
 
 - https://stackoverflow.com/questions/24718706/backup-restore-a-dockerized-postgresql-database