messages: Optimize get_recent_private_conversations.

Previously, get_recent_private_messages could take 100ms-1s to run,
contributing a substantial portion of the total runtime of `/`.

We fix this by taking advantage of the recent denormalization of
personal_recipient into the UserProfile model, allowing us to avoid
the complex join with Recipient that was previously required.

The change that requires additional commentary is the change to the
main, big SQL query:
1. We eliminate UserMessage table from the query, because the condition
m.recipient_id=%(my_recipient_id)d
implies m is a personal message to the user being processed - so joining
with usermessage to check for user_profile_id and flags&2048 (which
checks the message is private) is redundant.
2. We only need to join the Message table with UserProfile
(on sender_id) and get the sender's personal_recipient_id from their
UserProfile row.

Fixes #13437.

zerver/lib/message.py

@@ -1033,11 +1033,9 @@ def get_recent_conversations_recipient_id(user_profile: UserProfile,
     get_recent_private_conversations would have used to record that
     message in its data structure.
     """
-    my_recipient_id = Recipient.objects.get(type=Recipient.PERSONAL,
+    my_recipient_id = user_profile.id
-                                            type_id=user_profile.id).id
     if recipient_id == my_recipient_id:
-        return Recipient.objects.get(type=Recipient.PERSONAL,
+        return UserProfile.objects.values_list('recipient_id', flat=True).get(id=sender_id)
-                                     type_id=sender_id).id
     return recipient_id
 
 def get_recent_private_conversations(user_profile: UserProfile) -> Dict[int, Dict[str, Any]]:
@@ -1071,8 +1069,7 @@ def get_recent_private_conversations(user_profile: UserProfile) -> Dict[int, Dic
     RECENT_CONVERSATIONS_LIMIT = 1000
 
     recipient_map = {}
-    my_recipient_id = Recipient.objects.get(type=Recipient.PERSONAL,
+    my_recipient_id = user_profile.recipient_id
-                                            type_id=user_profile.id).id
 
     query = '''
     SELECT
@@ -1095,22 +1092,15 @@ def get_recent_private_conversations(user_profile: UserProfile) -> Dict[int, Dic
         LIMIT %(conversation_limit)d)
         UNION ALL
         (SELECT
-            um.message_id AS message_id,
+            m.id AS message_id,
-            r.id AS recipient_id
+            sender_profile.recipient_id AS recipient_id
         FROM
-            zerver_usermessage um
-        JOIN
             zerver_message m
-        ON
-            um.message_id = m.id
         JOIN
-            zerver_recipient r
+            zerver_userprofile sender_profile
         ON
-            r.type = 1 AND
+            m.sender_id = sender_profile.id
-            r.type_id = m.sender_id
         WHERE
-            um.user_profile_id=%(user_profile_id)d AND
-            um.flags & 2048 <> 0 AND
             m.recipient_id=%(my_recipient_id)d
         ORDER BY message_id DESC
         LIMIT %(conversation_limit)d)

zerver/tests/test_events.py

@@ -3505,7 +3505,7 @@ class FetchQueriesTest(ZulipTestCase):
                     client_gravatar=False,
                 )
 
-        self.assert_length(queries, 33)
+        self.assert_length(queries, 32)
 
         expected_counts = dict(
             alert_words=0,
@@ -3526,7 +3526,7 @@ class FetchQueriesTest(ZulipTestCase):
             realm_filters=1,
             realm_user=3,
             realm_user_groups=2,
-            recent_private_conversations=2,
+            recent_private_conversations=1,
             starred_messages=1,
             stream=2,
             stop_words=0,

zerver/tests/test_home.py

@@ -245,7 +245,7 @@ class HomeTest(ZulipTestCase):
         self.assertEqual(set(result["Cache-Control"].split(", ")),
                          {"must-revalidate", "no-store", "no-cache"})
 
-        self.assert_length(queries, 45)
+        self.assert_length(queries, 44)
         self.assert_length(cache_mock.call_args_list, 7)
 
         html = result.content.decode('utf-8')
@@ -311,7 +311,7 @@ class HomeTest(ZulipTestCase):
                 result = self._get_home_page()
                 self.assertEqual(result.status_code, 200)
                 self.assert_length(cache_mock.call_args_list, 6)
-            self.assert_length(queries, 42)
+            self.assert_length(queries, 41)
 
     @slow("Creates and subscribes 10 users in a loop.  Should use bulk queries.")
     def test_num_queries_with_streams(self) -> None:
@@ -343,7 +343,7 @@ class HomeTest(ZulipTestCase):
         with queries_captured() as queries2:
             result = self._get_home_page()
 
-        self.assert_length(queries2, 39)
+        self.assert_length(queries2, 38)
 
         # Do a sanity check that our new streams were in the payload.
         html = result.content.decode('utf-8')