code: func (e *SortExec) Open(ctx context.Context) error {
  	e.fetched = false
  	e.Idx = 0
  
  	// To avoid duplicated initialization for TopNExec.
  	if e.memTracker == nil {
  		e.memTracker = memory.NewTracker(e.id, e.ctx.GetSessionVars().MemQuotaSort)
  		e.memTracker.AttachTo(e.ctx.GetSessionVars().StmtCtx.MemTracker)
  	}
  	return e.children[0].Open(ctx)
  }
docs: // Open implements the Executor Open interface.