howto/upgrades.md

@@ -190,15 +190,23 @@ If you want to know which packages are marked as obsolete, you can use the
 following two commands:
 
     apt list "?obsolete"
-    # The following will mainly print the same list as above, but there might be
-    # some more in its list.
+    apt list "?narrow(?installed, ?not(?codename($(lsb_release -c -s | tail -1))))"
+
+The authoritative source for the alert, however, is the `apt_info`
+collector, which might differ from the above:
+
     DEBUG=1 /usr/share/prometheus-node-exporter-collectors/apt_info.py >/dev/null
 
 Check the state of each package with `apt policy $package` to determine what
 needs to be done with it. If most cases, the packages can just be purged, but
-maybe not if they are obsolete because an apt source was lost. In that latter
-case, you may want to check out why the source was removed and make sure to
-bring it back.
+maybe not if they are obsolete because an apt source was lost. 
+
+In that latter case, you may want to check out why the source was
+removed and make sure to bring it back. Sometimes it means downgrading
+the package to an earlier version, in case we used an incorrect
+backport (`apt.postgresql.org` packages, suffixed with `pgdg` are in
+that situation, as their version is higher than `debian.org`
+packages).
 
 ## Out of date package lists