code: func (e *TopNExec) Open(ctx context.Context) error {
  	e.memTracker = memory.NewTracker(e.id, e.ctx.GetSessionVars().MemQuotaTopn)
  	e.memTracker.AttachTo(e.ctx.GetSessionVars().StmtCtx.MemTracker)
  	return e.SortExec.Open(ctx)
  }
docs: // Open implements the Executor Open interface.