diff --git a/synapse/storage/database.py b/synapse/storage/database.py
index 41f566b6487a..5ddb58a8a2ca 100644
--- a/synapse/storage/database.py
+++ b/synapse/storage/database.py
@@ -31,6 +31,7 @@
     List,
     Optional,
     Tuple,
+    Type,
     TypeVar,
     cast,
     overload,
@@ -41,6 +42,7 @@
 from typing_extensions import Concatenate, Literal, ParamSpec
 
 from twisted.enterprise import adbapi
+from twisted.internet.interfaces import IReactorCore
 
 from synapse.api.errors import StoreError
 from synapse.config.database import DatabaseConnectionConfig
@@ -92,7 +94,9 @@
 
 
 def make_pool(
-    reactor, db_config: DatabaseConnectionConfig, engine: BaseDatabaseEngine
+    reactor: IReactorCore,
+    db_config: DatabaseConnectionConfig,
+    engine: BaseDatabaseEngine,
 ) -> adbapi.ConnectionPool:
     """Get the connection pool for the database."""
 
@@ -101,7 +105,7 @@ def make_pool(
     db_args = dict(db_config.config.get("args", {}))
     db_args.setdefault("cp_reconnect", True)
 
-    def _on_new_connection(conn):
+    def _on_new_connection(conn: Connection) -> None:
         # Ensure we have a logging context so we can correctly track queries,
         # etc.
         with LoggingContext("db.on_new_connection"):
@@ -157,7 +161,11 @@ class LoggingDatabaseConnection:
     default_txn_name: str
 
     def cursor(
-        self, *, txn_name=None, after_callbacks=None, exception_callbacks=None
+        self,
+        *,
+        txn_name: Optional[str] = None,
+        after_callbacks: Optional[List["_CallbackListEntry"]] = None,
+        exception_callbacks: Optional[List["_CallbackListEntry"]] = None,
     ) -> "LoggingTransaction":
         if not txn_name:
             txn_name = self.default_txn_name
@@ -183,11 +191,16 @@ def __enter__(self) -> "LoggingDatabaseConnection":
         self.conn.__enter__()
         return self
 
-    def __exit__(self, exc_type, exc_value, traceback) -> Optional[bool]:
+    def __exit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_value: Optional[BaseException],
+        traceback: Optional[types.TracebackType],
+    ) -> Optional[bool]:
         return self.conn.__exit__(exc_type, exc_value, traceback)
 
     # Proxy through any unknown lookups to the DB conn class.
-    def __getattr__(self, name):
+    def __getattr__(self, name: str) -> Any:
         return getattr(self.conn, name)
 
 
@@ -391,17 +404,22 @@ def close(self) -> None:
     def __enter__(self) -> "LoggingTransaction":
         return self
 
-    def __exit__(self, exc_type, exc_value, traceback):
+    def __exit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_value: Optional[BaseException],
+        traceback: Optional[types.TracebackType],
+    ) -> None:
         self.close()
 
 
 class PerformanceCounters:
-    def __init__(self):
-        self.current_counters = {}
-        self.previous_counters = {}
+    def __init__(self) -> None:
+        self.current_counters: Dict[str, Tuple[int, float]] = {}
+        self.previous_counters: Dict[str, Tuple[int, float]] = {}
 
     def update(self, key: str, duration_secs: float) -> None:
-        count, cum_time = self.current_counters.get(key, (0, 0))
+        count, cum_time = self.current_counters.get(key, (0, 0.0))
         count += 1
         cum_time += duration_secs
         self.current_counters[key] = (count, cum_time)
@@ -527,7 +545,7 @@ async def _check_safe_to_upsert(self) -> None:
     def start_profiling(self) -> None:
         self._previous_loop_ts = monotonic_time()
 
-        def loop():
+        def loop() -> None:
             curr = self._current_txn_total_time
             prev = self._previous_txn_total_time
             self._previous_txn_total_time = curr
@@ -1186,7 +1204,7 @@ def simple_upsert_txn_emulated(
         if lock:
             self.engine.lock_table(txn, table)
 
-        def _getwhere(key):
+        def _getwhere(key: str) -> str:
             # If the value we're passing in is None (aka NULL), we need to use
             # IS, not =, as NULL = NULL equals NULL (False).
             if keyvalues[key] is None:
@@ -2258,7 +2276,7 @@ async def simple_search_list(
         term: Optional[str],
         col: str,
         retcols: Collection[str],
-        desc="simple_search_list",
+        desc: str = "simple_search_list",
     ) -> Optional[List[Dict[str, Any]]]:
         """Executes a SELECT query on the named table, which may return zero or
         more rows, returning the result as a list of dicts.