code: func SupportStreaming(p PhysicalPlan) bool {
  	switch p.(type) {
  	case *PhysicalTableScan, *PhysicalIndexScan, *PhysicalSelection:
  		return true
  	}
  	return false
  }
docs: // SupportStreaming returns true if a pushed down operation supports using coprocessor streaming API.
  // Note that this function handle pushed down physical plan only! It's called in constructDAGReq.
  // Some plans are difficult (if possible) to implement streaming, and some are pointless to do so.
  // TODO: Support more kinds of physical plan.