planner: fix tidb_enforce_mpp cannot work

Signed-off-by: Weizhen Wang <wangweizhen@pingcap.com>

Author: hawkingrei

pkg/planner/core/logical_plan_builder.go

@@ -522,7 +522,7 @@ func setPreferredStoreType(ds *logicalop.DataSource, hintInfo *h.PlanHints) {
 			ds.SCtx().GetSessionVars().RaiseWarningWhenMPPEnforced("MPP mode may be blocked because you have set a hint to read table `" + hintTbl.TblName.O + "` from TiKV.")
 		}
 	}
-	if hintTbl := hintInfo.IfPreferTiFlash(alias); hintTbl != nil || ds.SCtx().GetSessionVars().IsMPPEnforced() {
+	if hintTbl := hintInfo.IfPreferTiFlash(alias); hintTbl != nil {
 		// `ds.PreferStoreType != 0`, which means there's a hint hit the both TiKV value and TiFlash value for table.
 		// We can't support read a table from two different storages, even partition table.
 		if ds.PreferStoreType != 0 {
@@ -535,9 +535,7 @@ func setPreferredStoreType(ds *logicalop.DataSource, hintInfo *h.PlanHints) {
 		for _, path := range ds.AllPossibleAccessPaths {
 			if path.StoreType == kv.TiFlash {
 				ds.PreferStoreType |= h.PreferTiFlash
-				if hintTbl != nil {
-					ds.PreferPartitions[h.PreferTiFlash] = hintTbl.Partitions
-				}
+				ds.PreferPartitions[h.PreferTiFlash] = hintTbl.Partitions
 				break
 			}
 		}

pkg/planner/core/plan_cost_ver2.go

@@ -724,13 +724,18 @@ func getPlanCostVer24PhysicalHashJoin(pp base.PhysicalPlan, taskType property.Ta
 	if taskType == property.MppTaskType { // BCast or Shuffle Join, use mppConcurrency
 		p.PlanCostVer2 = costusage.SumCostVer2(buildChildCost, probeChildCost,
 			costusage.DivCostVer2(costusage.SumCostVer2(buildHashCost, buildFilterCost, probeHashCost, probeFilterCost), mppConcurrency))
+		if p.SCtx().GetSessionVars().IsMPPEnforced() &&
+			!hasCostFlag(option.CostFlag, costusage.CostFlagRecalculate) { // show the real cost in explain-statements
+			p.PlanCostVer2 = costusage.DivCostVer2(p.PlanCostVer2, 1000000000)
+		}
 	} else { // TiDB HashJoin
 		startCost := costusage.NewCostVer2(option, cpuFactor,
 			10*3*cpuFactor.Value, // 10rows * 3func * cpuFactor
 			func() string { return fmt.Sprintf("cpu(10*3*%v)", cpuFactor) })
 		p.PlanCostVer2 = costusage.SumCostVer2(startCost, buildChildCost, probeChildCost, buildHashCost, buildFilterCost,
 			costusage.DivCostVer2(costusage.SumCostVer2(probeFilterCost, probeHashCost), tidbConcurrency))
 	}
+
 	p.PlanCostInit = true
 	// Multiply by cost factor - defaults to 1, but can be increased/decreased to influence the cost model
 	p.PlanCostVer2 = costusage.MulCostVer2(p.PlanCostVer2, p.SCtx().GetSessionVars().HashJoinCostFactor)