planner: introduce new cost formula for MPPAggs (pingcap#35436)

ref pingcap#35240

planner/core/plan_cost.go

@@ -312,7 +312,12 @@ func (p *PhysicalTableReader) GetPlanCost(taskType property.TaskType, costFlag u
 			concurrency = float64(p.ctx.GetSessionVars().DistSQLScanConcurrency())
 			rowSize = getTblStats(p.tablePlan).GetAvgRowSize(p.ctx, p.tablePlan.Schema().Columns, false, false)
 			seekCost = estimateNetSeekCost(p.tablePlan)
-			childCost, err := p.tablePlan.GetPlanCost(property.CopSingleReadTaskType, costFlag)
+			tType := property.MppTaskType
+			if p.ctx.GetSessionVars().CostModelVersion == modelVer1 {
+				// regard the underlying tasks as cop-task on modelVer1 for compatibility
+				tType = property.CopSingleReadTaskType
+			}
+			childCost, err := p.tablePlan.GetPlanCost(tType, costFlag)
 			if err != nil {
 				return 0, err
 			}
@@ -326,7 +331,8 @@ func (p *PhysicalTableReader) GetPlanCost(taskType property.TaskType, costFlag u
 		// consider concurrency
 		p.planCost /= concurrency
 		// consider tidb_enforce_mpp
-		if isMPP && p.ctx.GetSessionVars().IsMPPEnforced() {
+		if isMPP && p.ctx.GetSessionVars().IsMPPEnforced() &&
+			!hasCostFlag(costFlag, CostFlagRecalculate) { // show the real cost in explain-statements
 			p.planCost /= 1000000000
 		}
 	}
@@ -892,12 +898,19 @@ func (p *PhysicalHashJoin) GetPlanCost(taskType property.TaskType, costFlag uint
 }
 
 // GetCost computes cost of stream aggregation considering CPU/memory.
-func (p *PhysicalStreamAgg) GetCost(inputRows float64, isRoot bool, costFlag uint64) float64 {
+func (p *PhysicalStreamAgg) GetCost(inputRows float64, isRoot, isMPP bool, costFlag uint64) float64 {
 	aggFuncFactor := p.getAggFuncCostFactor(false)
 	var cpuCost float64
 	sessVars := p.ctx.GetSessionVars()
 	if isRoot {
 		cpuCost = inputRows * sessVars.GetCPUFactor() * aggFuncFactor
+	} else if isMPP {
+		if p.ctx.GetSessionVars().CostModelVersion == modelVer2 {
+			// use the dedicated CPU factor for TiFlash on modelVer2
+			cpuCost = inputRows * sessVars.GetTiFlashCPUFactor() * aggFuncFactor
+		} else {
+			cpuCost = inputRows * sessVars.GetCopCPUFactor() * aggFuncFactor
+		}
 	} else {
 		cpuCost = inputRows * sessVars.GetCopCPUFactor() * aggFuncFactor
 	}
@@ -916,7 +929,7 @@ func (p *PhysicalStreamAgg) GetPlanCost(taskType property.TaskType, costFlag uin
 		return 0, err
 	}
 	p.planCost = childCost
-	p.planCost += p.GetCost(getCardinality(p.children[0], costFlag), taskType == property.RootTaskType, costFlag)
+	p.planCost += p.GetCost(getCardinality(p.children[0], costFlag), taskType == property.RootTaskType, taskType == property.MppTaskType, costFlag)
 	p.planCostInit = true
 	return p.planCost, nil
 }
@@ -936,6 +949,13 @@ func (p *PhysicalHashAgg) GetCost(inputRows float64, isRoot, isMPP bool, costFla
 			// Cost of additional goroutines.
 			cpuCost += (con + 1) * sessVars.GetConcurrencyFactor()
 		}
+	} else if isMPP {
+		if p.ctx.GetSessionVars().CostModelVersion == modelVer2 {
+			// use the dedicated CPU factor for TiFlash on modelVer2
+			cpuCost = inputRows * sessVars.GetTiFlashCPUFactor() * aggFuncFactor
+		} else {
+			cpuCost = inputRows * sessVars.GetCopCPUFactor() * aggFuncFactor
+		}
 	} else {
 		cpuCost = inputRows * sessVars.GetCopCPUFactor() * aggFuncFactor
 	}
@@ -1144,6 +1164,9 @@ func (p *PhysicalExchangeReceiver) GetPlanCost(taskType property.TaskType, costF
 }
 
 func getOperatorActRows(operator PhysicalPlan) float64 {
+	if operator == nil {
+		return 0
+	}
 	runtimeInfo := operator.SCtx().GetSessionVars().StmtCtx.RuntimeStatsColl
 	id := operator.ID()
 	actRows := 0.0

planner/core/task.go

@@ -1666,7 +1666,7 @@ func (p *PhysicalStreamAgg) attach2Task(tasks ...task) task {
 					partialAgg.SetChildren(cop.indexPlan)
 					cop.indexPlan = partialAgg
 				}
-				cop.addCost(partialAgg.(*PhysicalStreamAgg).GetCost(inputRows, false, 0))
+				cop.addCost(partialAgg.(*PhysicalStreamAgg).GetCost(inputRows, false, false, 0))
 				partialAgg.SetCost(cop.cost())
 			}
 			t = cop.convertToRootTask(p.ctx)
@@ -1679,7 +1679,7 @@ func (p *PhysicalStreamAgg) attach2Task(tasks ...task) task {
 	} else {
 		attachPlan2Task(p, t)
 	}
-	t.addCost(final.GetCost(inputRows, true, 0))
+	t.addCost(final.GetCost(inputRows, true, false, 0))
 	t.plan().SetCost(t.cost())
 	return t
 }