planner: update the cost formula of IndexJoin+IndexLookup (#39617)

ref #35240
pingcap · Dec 6, 2022 · 1ddc592 · 1ddc592
1 parent 46ee6af
commit 1ddc592
Showing 1 changed file with 5 additions and 0 deletions.
diff --git a/planner/core/plan_cost_ver2.go b/planner/core/plan_cost_ver2.go
@@ -593,6 +593,11 @@ func (p *PhysicalIndexJoin) getIndexJoinCostVer2(taskType property.TaskType, opt
 	// Use an empirical value batchRatio to handle this now.
 	// TODO: remove this empirical value.
 	batchRatio := 6.0
+	if _, isIndexLookUp := probe.(*PhysicalIndexLookUpReader); isIndexLookUp {
+		// IndexJoin + IndexLookUp has the risks to cause massive double-read requests,
+		// set the batchRatio to a lower value in this case for safety.
+		batchRatio = 1.5
+	}
 	probeCost := divCostVer2(mulCostVer2(probeChildCost, buildRows), batchRatio)
 
 	p.planCostVer2 = sumCostVer2(startCost, buildChildCost, buildFilterCost, buildTaskCost, divCostVer2(sumCostVer2(probeCost, probeFilterCost, hashTableCost), probeConcurrency))