planner: prefer to use TiKV to process small table scans and consider IndexJoin batch size in cost model ver2 (#36690)

close pingcap/tidb#36635, close pingcap/tidb#36668

planner/core/plan_cost.go

@@ -443,6 +443,11 @@ func (p *PhysicalTableScan) GetPlanCost(taskType property.TaskType, costFlag uin
 		rowSize := math.Max(p.getScanRowSize(), 2.0) // to guarantee logRowSize >= 1
 		logRowSize := math.Log2(rowSize)
 		selfCost = getCardinality(p, costFlag) * logRowSize * scanFactor
+
+		// give TiFlash a start-up cost to let the optimizer prefers to use TiKV to process small table scans.
+		if p.StoreType == kv.TiFlash {
+			selfCost += 2000 * logRowSize * scanFactor
+		}
 	}
 
 	p.planCost = selfCost
@@ -528,6 +533,14 @@ func (p *PhysicalIndexJoin) GetCost(outerCnt, innerCnt, outerCost, innerCost flo
 	memoryCost := innerConcurrency * (batchSize * distinctFactor) * innerCnt * sessVars.GetMemoryFactor()
 	// Cost of inner child plan, i.e, mainly I/O and network cost.
 	innerPlanCost := outerCnt * innerCost
+	if p.ctx.GetSessionVars().CostModelVersion == 2 {
+		// IndexJoin executes a batch of rows at a time, so the actual cost of this part should be
+		//  `innerCostPerBatch * numberOfBatches` instead of `innerCostPerRow * numberOfOuterRow`.
+		// Use an empirical value batchRatio to handle this now.
+		// TODO: remove this empirical value.
+		batchRatio := 30.0
+		innerPlanCost /= batchRatio
+	}
 	return outerCost + innerPlanCost + cpuCost + memoryCost + p.estDoubleReadCost(outerCnt)
 }
 

planner/core/plan_cost_test.go

@@ -1004,3 +1004,39 @@ func TestIssue36243(t *testing.T) {
 	selCost, readerCost = getCost()
 	require.True(t, selCost > readerCost)
 }
+
+func TestScanOnSmallTable(t *testing.T) {
+	store, clean := testkit.CreateMockStore(t)
+	defer clean()
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec(`create table t (a int)`)
+	tk.MustExec("insert into t values (1), (2), (3), (4), (5)")
+	tk.MustExec("analyze table t")
+	tk.MustExec(`set @@tidb_cost_model_version=2`)
+
+	// Create virtual tiflash replica info.
+	dom := domain.GetDomain(tk.Session())
+	is := dom.InfoSchema()
+	db, exists := is.SchemaByName(model.NewCIStr("test"))
+	require.True(t, exists)
+	for _, tblInfo := range db.Tables {
+		if tblInfo.Name.L == "t" {
+			tblInfo.TiFlashReplica = &model.TiFlashReplicaInfo{
+				Count:     1,
+				Available: true,
+			}
+		}
+	}
+
+	rs := tk.MustQuery("explain select * from t").Rows()
+	useTiKVScan := false
+	for _, r := range rs {
+		op := r[0].(string)
+		task := r[2].(string)
+		if strings.Contains(op, "Scan") && strings.Contains(task, "tikv") {
+			useTiKVScan = true
+		}
+	}
+	require.True(t, useTiKVScan)
+}