skip some dict munging in event persistence (#11560)

Create a new dict helper method `simple_insert_many_values_txn`, which takes
raw row values, rather than {key=>value} dicts. This saves us a bunch of dict
munging, and makes it easier to use generators rather than creating
intermediate lists and dicts.

synapse/storage/database.py

@@ -896,6 +896,9 @@ class DatabasePool:
     ) -> None:
         """Executes an INSERT query on the named table.
 
+        The input is given as a list of dicts, with one dict per row.
+        Generally simple_insert_many_values should be preferred for new code.
+
         Args:
             table: string giving the table name
             values: dict of new column names and values for them
@@ -909,6 +912,9 @@ class DatabasePool:
     ) -> None:
         """Executes an INSERT query on the named table.
 
+        The input is given as a list of dicts, with one dict per row.
+        Generally simple_insert_many_values_txn should be preferred for new code.
+
         Args:
             txn: The transaction to use.
             table: string giving the table name
@@ -933,23 +939,66 @@ class DatabasePool:
             if k != keys[0]:
                 raise RuntimeError("All items must have the same keys")
 
+        return DatabasePool.simple_insert_many_values_txn(txn, table, keys[0], vals)
+
+    async def simple_insert_many_values(
+        self,
+        table: str,
+        keys: Collection[str],
+        values: Iterable[Iterable[Any]],
+        desc: str,
+    ) -> None:
+        """Executes an INSERT query on the named table.
+
+        The input is given as a list of rows, where each row is a list of values.
+        (Actually any iterable is fine.)
+
+        Args:
+            table: string giving the table name
+            keys: list of column names
+            values: for each row, a list of values in the same order as `keys`
+            desc: description of the transaction, for logging and metrics
+        """
+        await self.runInteraction(
+            desc, self.simple_insert_many_values_txn, table, keys, values
+        )
+
+    @staticmethod
+    def simple_insert_many_values_txn(
+        txn: LoggingTransaction,
+        table: str,
+        keys: Collection[str],
+        values: Iterable[Iterable[Any]],
+    ) -> None:
+        """Executes an INSERT query on the named table.
+
+        The input is given as a list of rows, where each row is a list of values.
+        (Actually any iterable is fine.)
+
+        Args:
+            txn: The transaction to use.
+            table: string giving the table name
+            keys: list of column names
+            values: for each row, a list of values in the same order as `keys`
+        """
+
         if isinstance(txn.database_engine, PostgresEngine):
             # We use `execute_values` as it can be a lot faster than `execute_batch`,
             # but it's only available on postgres.
             sql = "INSERT INTO %s (%s) VALUES ?" % (
                 table,
-                ", ".join(k for k in keys[0]),
+                ", ".join(k for k in keys),
             )
 
-            txn.execute_values(sql, vals, fetch=False)
+            txn.execute_values(sql, values, fetch=False)
         else:
             sql = "INSERT INTO %s (%s) VALUES(%s)" % (
                 table,
-                ", ".join(k for k in keys[0]),
-                ", ".join("?" for _ in keys[0]),
+                ", ".join(k for k in keys),
+                ", ".join("?" for _ in keys),
             )
 
-            txn.execute_batch(sql, vals)
+            txn.execute_batch(sql, values)
 
     async def simple_upsert(
         self,