Annotate the removed indicies with why they were removed.

synapse/storage/schema/delta/32/remove_indices.sql

@@ -16,27 +16,24 @@
 
 -- The following indices are redundant, other indices are equivalent or
 -- supersets
-DROP INDEX IF EXISTS events_room_id;
-DROP INDEX IF EXISTS events_order;
-DROP INDEX IF EXISTS events_topological_ordering;
-DROP INDEX IF EXISTS events_stream_ordering;
-DROP INDEX IF EXISTS state_groups_id;
-DROP INDEX IF EXISTS event_to_state_groups_id;
-DROP INDEX IF EXISTS event_push_actions_room_id_event_id_user_id_profile_tag;
-DROP INDEX IF EXISTS event_push_actions_room_id_user_id;
-
-DROP INDEX IF EXISTS event_destinations_id;
-DROP INDEX IF EXISTS st_extrem_id;
-DROP INDEX IF EXISTS event_content_hashes_id;
-DROP INDEX IF EXISTS event_signatures_id;
-DROP INDEX IF EXISTS event_edge_hashes_id;
-DROP INDEX IF EXISTS redactions_event_id;
-DROP INDEX IF EXISTS remote_media_cache_thumbnails_media_id;
-DROP INDEX IF EXISTS room_hosts_room_id;
-DROP INDEX IF EXISTS event_search_ev_ridx;
+DROP INDEX IF EXISTS events_room_id; -- Prefix of events_room_stream
+DROP INDEX IF EXISTS events_order; -- Prefix of events_order_topo_stream_room
+DROP INDEX IF EXISTS events_topological_ordering; -- Prefix of events_order_topo_stream_room
+DROP INDEX IF EXISTS events_stream_ordering; -- Duplicate of PRIMARY KEY
+DROP INDEX IF EXISTS state_groups_id; -- Duplicate of PRIMARY KEY
+DROP INDEX IF EXISTS event_to_state_groups_id; -- Duplicate of PRIMARY KEY
+DROP INDEX IF EXISTS event_push_actions_room_id_event_id_user_id_profile_tag; -- Duplicate of UNIQUE CONSTRAINT
 
+DROP INDEX IF EXISTS event_destinations_id; -- Prefix of UNIQUE CONSTRAINT
+DROP INDEX IF EXISTS st_extrem_id; -- Prefix of UNIQUE CONSTRAINT
+DROP INDEX IF EXISTS event_content_hashes_id; -- Prefix of UNIQUE CONSTRAINT
+DROP INDEX IF EXISTS event_signatures_id; -- Prefix of UNIQUE CONSTRAINT
+DROP INDEX IF EXISTS event_edge_hashes_id; -- Prefix of UNIQUE CONSTRAINT
+DROP INDEX IF EXISTS redactions_event_id; -- Duplicate of UNIQUE CONSTRAINT
+DROP INDEX IF EXISTS room_hosts_room_id; -- Prefix of UNIQUE CONSTRAINT
 
 -- The following indices were unused
+DROP INDEX IF EXISTS remote_media_cache_thumbnails_media_id;
 DROP INDEX IF EXISTS evauth_edges_auth_id;
 DROP INDEX IF EXISTS topics_room_id;
 DROP INDEX IF EXISTS presence_stream_state;