Remove manys calls to cursor_to_dict (#16431)

This avoids calling cursor_to_dict and then immediately
unpacking the values in the dict for other users. By not
creating the intermediate dictionary we can avoid allocating
the dictionary and strings for the keys, which should generally
be more performant.

Additionally this improves type hints by avoid Dict[str, Any]
dictionaries coming out of the database layer.

1 files changed, 19 insertions, 23 deletions

diff --git a/synapse/storage/databases/main/room.py b/synapse/storage/databases/main/room.py

index 719e11aea6..1d4d99932b 100644
--- a/synapse/storage/databases/main/room.py
+++ b/synapse/storage/databases/main/room.py
@@ -831,7 +831,7 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
 
         def get_retention_policy_for_room_txn(
             txn: LoggingTransaction,
-        ) -> List[Dict[str, Optional[int]]]:
+        ) -> Optional[Tuple[Optional[int], Optional[int]]]:
             txn.execute(
                 """
                 SELECT min_lifetime, max_lifetime FROM room_retention
@@ -841,7 +841,7 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
                 (room_id,),
             )
 
-            return self.db_pool.cursor_to_dict(txn)
+            return cast(Optional[Tuple[Optional[int], Optional[int]]], txn.fetchone())
 
         ret = await self.db_pool.runInteraction(
             "get_retention_policy_for_room",
@@ -856,8 +856,7 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
                 max_lifetime=self.config.retention.retention_default_max_lifetime,
             )
 
-        min_lifetime = ret[0]["min_lifetime"]
-        max_lifetime = ret[0]["max_lifetime"]
+        min_lifetime, max_lifetime = ret
 
         # If one of the room's policy's attributes isn't defined, use the matching
         # attribute from the default policy.
@@ -1162,14 +1161,13 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
 
             txn.execute(sql, args)
 
-            rows = self.db_pool.cursor_to_dict(txn)
-            rooms_dict = {}
-
-            for row in rows:
-                rooms_dict[row["room_id"]] = RetentionPolicy(
-                    min_lifetime=row["min_lifetime"],
-                    max_lifetime=row["max_lifetime"],
+            rooms_dict = {
+                room_id: RetentionPolicy(
+                    min_lifetime=min_lifetime,
+                    max_lifetime=max_lifetime,
                 )
+                for room_id, min_lifetime, max_lifetime in txn
+            }
 
             if include_null:
                 # If required, do a second query that retrieves all of the rooms we know
@@ -1178,13 +1176,11 @@ class RoomWorkerStore(CacheInvalidationWorkerStore):
 
                 txn.execute(sql)
 
-                rows = self.db_pool.cursor_to_dict(txn)
-
                 # If a room isn't already in the dict (i.e. it doesn't have a retention
                 # policy in its state), add it with a null policy.
-                for row in rows:
-                    if row["room_id"] not in rooms_dict:
-                        rooms_dict[row["room_id"]] = RetentionPolicy()
+                for (room_id,) in txn:
+                    if room_id not in rooms_dict:
+                        rooms_dict[room_id] = RetentionPolicy()
 
             return rooms_dict
 
@@ -1703,24 +1699,24 @@ class RoomBackgroundUpdateStore(SQLBaseStore):
                 (last_room, batch_size),
             )
 
-            rows = self.db_pool.cursor_to_dict(txn)
+            rows = txn.fetchall()
 
             if not rows:
                 return True
 
-            for row in rows:
-                if not row["json"]:
+            for room_id, event_id, json in rows:
+                if not json:
                     retention_policy = {}
                 else:
-                    ev = db_to_json(row["json"])
+                    ev = db_to_json(json)
                     retention_policy = ev["content"]
 
                 self.db_pool.simple_insert_txn(
                     txn=txn,
                     table="room_retention",
                     values={
-                        "room_id": row["room_id"],
-                        "event_id": row["event_id"],
+                        "room_id": room_id,
+                        "event_id": event_id,
                         "min_lifetime": retention_policy.get("min_lifetime"),
                         "max_lifetime": retention_policy.get("max_lifetime"),
                     },
@@ -1729,7 +1725,7 @@ class RoomBackgroundUpdateStore(SQLBaseStore):
             logger.info("Inserted %d rows into room_retention", len(rows))
 
             self.db_pool.updates._background_update_progress_txn(
-                txn, "insert_room_retention", {"room_id": rows[-1]["room_id"]}
+                txn, "insert_room_retention", {"room_id": rows[-1][0]}
             )
 
             if batch_size > len(rows):