User Cursor.__iter__ instead of fetchall

This prevents unnecessary construction of lists

synapse/storage/_base.py

@@ -73,6 +73,9 @@ class LoggingTransaction(object):
     def __setattr__(self, name, value):
         setattr(self.txn, name, value)
 
+    def __iter__(self):
+        return self.txn.__iter__()
+
     def execute(self, sql, *args):
         self._do_execute(self.txn.execute, sql, *args)
 
@@ -357,7 +360,7 @@ class SQLBaseStore(object):
         """
         col_headers = list(intern(column[0]) for column in cursor.description)
         results = list(
-            dict(zip(col_headers, row)) for row in cursor.fetchall()
+            dict(zip(col_headers, row)) for row in cursor
         )
         return results
 
@@ -579,7 +582,7 @@ class SQLBaseStore(object):
 
         txn.execute(sql, keyvalues.values())
 
-        return [r[0] for r in txn.fetchall()]
+        return [r[0] for r in txn]
 
     def _simple_select_onecol(self, table, keyvalues, retcol,
                               desc="_simple_select_onecol"):
@@ -901,14 +904,14 @@ class SQLBaseStore(object):
 
         txn = db_conn.cursor()
         txn.execute(sql, (int(max_value),))
-        rows = txn.fetchall()
-        txn.close()
 
         cache = {
             row[0]: int(row[1])
-            for row in rows
+            for row in txn
         }
 
+        txn.close()
+
         if cache:
             min_val = min(cache.values())
         else: