实现兼容MySQL SET自定义变量的功能

src/gausskernel/process/threadpool/threadpool_worker.cpp

@@ -819,6 +819,8 @@ static bool InitSession(knl_session_context* session)
     /* Init GUC option for this session. */
     InitializeGUCOptions();
 
+    init_set_user_params_htab();
+
     /* Read in remaining GUC variables */
     read_nondefault_variables();