topic: Add comments calling out case-sensitive index usage.

zerver/lib/topic.py

@@ -226,6 +226,9 @@ def generate_topic_history_from_db_rows(rows: List[Tuple[str, int]]) -> List[Dic
 
 def get_topic_history_for_public_stream(realm_id: int, recipient_id: int) -> List[Dict[str, Any]]:
     cursor = connection.cursor()
+    # Uses index: zerver_message_realm_recipient_subject
+    # Note that this is *case-sensitive*, so that we can display the
+    # most recently-used case (in generate_topic_history_from_db_rows)
     query = """
     SELECT
         "zerver_message"."subject" as topic,
@@ -254,6 +257,9 @@ def get_topic_history_for_stream(
         return get_topic_history_for_public_stream(user_profile.realm_id, recipient_id)
 
     cursor = connection.cursor()
+    # Uses index: zerver_message_realm_recipient_subject
+    # Note that this is *case-sensitive*, so that we can display the
+    # most recently-used case (in generate_topic_history_from_db_rows)
     query = """
     SELECT
         "zerver_message"."subject" as topic,

zerver/models.py

@@ -3175,7 +3175,9 @@ class Message(AbstractMessage):
                 name="zerver_message_realm_recipient_upper_subject",
             ),
             models.Index(
-                # Only used by already_sent_mirrored_message_id
+                # Used by already_sent_mirrored_message_id, and when
+                # determining recent topics (we post-process to merge
+                # and show the most recent case)
                 "realm_id",
                 "recipient_id",
                 "subject",