Rewrite highest priority rule txn to use FOR UPDATE in one query

synapse/storage/databases/main/push_rule.py

@@ -465,23 +465,18 @@ def _add_push_rule_highest_priority_txn(
         conditions_json: str,
         actions_json: str,
     ) -> None:
+        sql = """
+            SELECT COUNT(*), MAX(priority) FROM push_rules"
+            WHERE user_name = ? and priority_class = ?
+        """
+
         if isinstance(self.database_engine, PostgresEngine):
-            # Postgres doesn't do FOR SHARE on aggregate functions, so select the rows first
-            # then re-select the count/max below.
-            sql = """
-                SELECT * FROM push_rules
-                WHERE user_name = ? and priority_class = ?
-                FOR SHARE
-            """
+            sql += " FOR UPDATE"
         else:
             # Annoyingly SQLite doesn't support row level locking, so lock the whole table
             self.database_engine.lock_table(txn, "push_rules")
 
         # find the highest priority rule in that class
-        sql = (
-            "SELECT COUNT(*), MAX(priority) FROM push_rules"
-            " WHERE user_name = ? and priority_class = ?"
-        )
         txn.execute(sql, (user_id, priority_class))
         res = txn.fetchall()
         (how_many, highest_prio) = res[0]