Update the query that fetches roles to be faster with many roles.

This changes the query that fetches roles from using the
pg_catalog.shobj_description function to using an explicit join on
pg_catalog.pg_shdescription. In my specific case, in a production system with
over 20k roles, this reduces the query time down from 8 minutes(!) down to
~40ms. Just in case it was a locking issue, I did use pg_activity to
investigate locks, and nothing was being blocked during the 8 minutes.

This fixes a problem that exists only for certain users, and specifically in my
case where the user that runs the query is themselves a member (directly and
indirectly) of a high number of roles. Running the existing query as a user
that has a low number of roles (and specifically, the master user in an AWS RDS
database), the existing query is fairly performant.

My best explanation as to the performance difference comes from the definition
of shobj_description
https://github.com/postgres/postgres/blob/2a8a00674e973dec4d7d74d5e16c7d7cdedf2be5/src/backend/catalog/system_functions.sql#L303-L312.
Each time it's called it runs a query against pg_shdescription and pg_class,
and so in my case runs 20k times, compared to the equivalent of once when doing
the explicit join on pg_shdescription. It should also be noted that in my case
pg_class is quite big - currently about 500k rows (although it has grown to ~5
million in the past, but that was borderline a mistake).

See #8457 (comment) for
EXPLAIN ANALYZE results of the existing query and the updated query.

Author: michalc

web/pgadmin/browser/server_groups/servers/roles/templates/roles/sql/default/nodes.sql

@@ -1,8 +1,8 @@
 SELECT
-	r.oid, r.rolname, r.rolcanlogin, r.rolsuper,
-	pg_catalog.shobj_description(r.oid, 'pg_authid') AS description
-FROM
-	pg_catalog.pg_roles r
+    r.oid, r.rolname, r.rolcanlogin, r.rolsuper, d.description
+FROM pg_catalog.pg_roles r
+    LEFT JOIN pg_catalog.pg_shdescription d
+    ON d.objoid = r.oid AND d.classoid = 'pg_catalog.pg_authid'::regclass
 {% if rid %}
 WHERE r.oid = {{ rid|qtLiteral(conn) }}::oid
 {% endif %}