planner: set the default of the tidb_cost_model_version correctly (#61608) (#62184)

close #61565

Author: ti-chi-bot

pkg/planner/core/plan_cost_ver1.go

@@ -29,6 +29,7 @@ import (
 	"github.com/pingcap/tidb/pkg/planner/util/optimizetrace"
 	"github.com/pingcap/tidb/pkg/sessionctx/variable"
 	"github.com/pingcap/tidb/pkg/statistics"
+	"github.com/pingcap/tidb/pkg/util/intest"
 	"github.com/pingcap/tidb/pkg/util/paging"
 )
 
@@ -87,6 +88,7 @@ func (p *PhysicalProjection) GetCost(count float64) float64 {
 
 // GetPlanCostVer1 calculates the cost of the plan if it has not been calculated yet and returns the cost.
 func (p *PhysicalProjection) GetPlanCostVer1(taskType property.TaskType, option *optimizetrace.PlanCostOption) (float64, error) {
+	intest.Assert(p.SCtx().GetSessionVars().CostModelVersion != 0)
 	costFlag := option.CostFlag
 	if p.PlanCostInit && !hasCostFlag(costFlag, costusage.CostFlagRecalculate) {
 		return p.PlanCost, nil

pkg/sessionctx/variable/session.go

@@ -2189,6 +2189,7 @@ func NewSessionVars(hctx HookContext) *SessionVars {
 		GroupConcatMaxLen:             DefGroupConcatMaxLen,
 		EnableRedactLog:               DefTiDBRedactLog,
 		EnableWindowFunction:          DefEnableWindowFunction,
+		CostModelVersion:              DefTiDBCostModelVer,
 	}
 	vars.status.Store(uint32(mysql.ServerStatusAutocommit))
 	vars.StmtCtx.ResourceGroupName = resourcegroup.DefaultResourceGroupName