From 5e17922ef715e3c68911955b32f42a70d6728831 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Tue, 31 May 2022 13:51:49 +0100
Subject: [PATCH] Stop reading from `event_edges.room_id`. (#12914)

event_edges.room_id is implied by the event id, so there is no need to join on the room id.
---
 changelog.d/12914.misc                        |  1 +
 .../databases/main/event_federation.py        |  7 +---
 .../storage/databases/main/events_worker.py   | 41 +++++++++----------
 synapse/storage/schema/__init__.py            |  5 ++-
 4 files changed, 26 insertions(+), 28 deletions(-)
 create mode 100644 changelog.d/12914.misc

diff --git a/changelog.d/12914.misc b/changelog.d/12914.misc
new file mode 100644
index 0000000000..07d819932a
--- /dev/null
+++ b/changelog.d/12914.misc
@@ -0,0 +1 @@
+Preparation for database schema simplifications: stop reading from `event_edges.room_id`.
diff --git a/synapse/storage/databases/main/event_federation.py b/synapse/storage/databases/main/event_federation.py
index 562dcbe94d..eec55b6478 100644
--- a/synapse/storage/databases/main/event_federation.py
+++ b/synapse/storage/databases/main/event_federation.py
@@ -1318,17 +1318,14 @@ class EventFederationWorkerStore(SignatureWorkerStore, EventsWorkerStore, SQLBas
 
         query = (
             "SELECT prev_event_id FROM event_edges "
-            "WHERE room_id = ? AND event_id = ? AND is_state = ? "
+            "WHERE event_id = ? AND NOT is_state "
             "LIMIT ?"
         )
 
         while front and len(event_results) < limit:
             new_front = set()
             for event_id in front:
-                txn.execute(
-                    query, (room_id, event_id, False, limit - len(event_results))
-                )
-
+                txn.execute(query, (event_id, limit - len(event_results)))
                 new_results = {t[0] for t in txn} - seen_events
 
                 new_front |= new_results
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index a97d7e1664..b99b107784 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -1928,23 +1928,6 @@ class EventsWorkerStore(SQLBaseStore):
                 LIMIT 1
             """
 
-            # Check to see whether the event in question is already referenced
-            # by another event. If we don't see any edges, we're next to a
-            # forward gap.
-            forward_edge_query = """
-                SELECT 1 FROM event_edges
-                /* Check to make sure the event referencing our event in question is not rejected */
-                LEFT JOIN rejections ON event_edges.event_id = rejections.event_id
-                WHERE
-                    event_edges.room_id = ?
-                    AND event_edges.prev_event_id = ?
-                    /* It's not a valid edge if the event referencing our event in
-                     * question is rejected.
-                     */
-                    AND rejections.event_id IS NULL
-                LIMIT 1
-            """
-
             # We consider any forward extremity as the latest in the room and
             # not a forward gap.
             #
@@ -1954,16 +1937,30 @@ class EventsWorkerStore(SQLBaseStore):
             # is useless. The new latest messages will just be federated as
             # usual.
             txn.execute(forward_extremity_query, (event.room_id, event.event_id))
-            forward_extremities = txn.fetchall()
-            if len(forward_extremities):
+            if txn.fetchone():
                 return False
 
+            # Check to see whether the event in question is already referenced
+            # by another event. If we don't see any edges, we're next to a
+            # forward gap.
+            forward_edge_query = """
+                SELECT 1 FROM event_edges
+                /* Check to make sure the event referencing our event in question is not rejected */
+                LEFT JOIN rejections ON event_edges.event_id = rejections.event_id
+                WHERE
+                    event_edges.prev_event_id = ?
+                    /* It's not a valid edge if the event referencing our event in
+                     * question is rejected.
+                     */
+                    AND rejections.event_id IS NULL
+                LIMIT 1
+            """
+
             # If there are no forward edges to the event in question (another
             # event hasn't referenced this event in their prev_events), then we
             # assume there is a forward gap in the history.
-            txn.execute(forward_edge_query, (event.room_id, event.event_id))
-            forward_edges = txn.fetchall()
-            if not len(forward_edges):
+            txn.execute(forward_edge_query, (event.event_id,))
+            if not txn.fetchone():
                 return True
 
             return False
diff --git a/synapse/storage/schema/__init__.py b/synapse/storage/schema/__init__.py
index da98f05e03..19466150d4 100644
--- a/synapse/storage/schema/__init__.py
+++ b/synapse/storage/schema/__init__.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-SCHEMA_VERSION = 70  # remember to update the list below when updating
+SCHEMA_VERSION = 71  # remember to update the list below when updating
 """Represents the expectations made by the codebase about the database schema
 
 This should be incremented whenever the codebase changes its requirements on the
@@ -67,6 +67,9 @@ Changes in SCHEMA_VERSION = 69:
 
 Changes in SCHEMA_VERSION = 70:
     - event_reference_hashes is no longer written to.
+
+Changes in SCHEMA_VERSION = 71:
+    - event_edges.room_id is no longer read from.
 """