Document how to administer synapse database via tunneled web-frontend

docs/maintenance-synapse.md

@@ -14,6 +14,7 @@ Table of contents:
 	- [Purging old data with the Purge History API](#purging-old-data-with-the-purge-history-api)
 	- [Compressing state with rust-synapse-compress-state](#compressing-state-with-rust-synapse-compress-state)
 
+- [Browse and manipulate the database](#browse-and-manipulate-the-database), for when you really need to take matters into your own hands
 
 ## Purging unused data with synapse-janitor
 
@@ -76,3 +77,23 @@ Don't forget that disk space only ever gets released after a [`FULL` Postgres `V
 Unfortunately, at this time the playbook can't help you run this **experimental tool**.
 
 Since it's also experimental, you may wish to stay away from it, or at least [make Postgres backups](./maintenance-postgres.md#backing-up-postgresql) first.
+
+## Browse and manipulate the database
+
+When the [matrix admin API](https://github.com/matrix-org/synapse/tree/master/docs/admin_api) and the other tools do not provide a more convenient way, having a look at synapse's postgresql database can satisfy a lot of admins' needs.
+First, set up an SSH tunnel to your matrix server (skip if it is your local machine):
+
+```
+# you may replace 1799 with an arbitrary port unbound on both machines
+ssh -L 1799:localhost:1799 matrix.DOMAIN
+```
+
+Then start up an ephemeral [adminer](https://www.adminer.org/) container, connecting it to the matrix-net and linking the postgresql container:
+
+```
+docker run --rm --publish 1799:8080 --link matrix-postgres --net matrix adminer
+```
+
+You should then be able to browse the adminer database administration GUI at http://localhost:1799/ after entering your DB credentials (found in the `host_vars` or on the server in `{{matrix_synapse_config_dir_path}}/homeserver.yaml` under `database.args`)
+
+⚠️ Be **very careful** with this, there is **no undo** for impromptu DB operations.