use prop to pass expectCnt, add tests

Signed-off-by: you06 <[email protected]>

planner/core/find_best_task.go

@@ -1216,6 +1216,7 @@ func (ds *DataSource) convertToIndexScan(prop *property.PhysicalProperty, candid
 		indexPlan:   is,
 		tblColHists: ds.TblColHists,
 		tblCols:     ds.TblCols,
+		expectCnt:   uint64(prop.ExpectedCnt),
 	}
 	cop.partitionInfo = PartitionInfo{
 		PruningConds:   ds.allConds,

planner/core/plan_test.go

@@ -668,6 +668,18 @@ func TestCopPaging(t *testing.T) {
 			"    └─TableRowIDScan 1024.00 cop[tikv] table:t keep order:false"))
 	}
 
+	// selection between limit and indexlookup, limit 960 should also go paging
+	for i := 0; i < 10; i++ {
+		tk.MustQuery("explain format='brief' select * from t force index(i) where mod(id, 2) > 0 and id <= 1024 and c1 >= 0 and c1 <= 1024 and c2 in (2, 4, 6, 8) order by c1 limit 960").Check(kit.Rows(
+			"Limit 3.20 root  offset:0, count:960",
+			"└─Selection 2.56 root  gt(mod(test.t.id, 2), 0)",
+			"  └─IndexLookUp 3.20 root  paging:true",
+			"    ├─Selection(Build) 819.20 cop[tikv]  le(test.t.id, 1024)",
+			"    │ └─IndexRangeScan 1024.00 cop[tikv] table:t, index:i(c1) range:[0,1024], keep order:true",
+			"    └─Selection(Probe) 3.20 cop[tikv]  in(test.t.c2, 2, 4, 6, 8)",
+			"      └─TableRowIDScan 819.20 cop[tikv] table:t keep order:false"))
+	}
+
 	// limit 961 exceeds the threshold, it should not go paging
 	for i := 0; i < 10; i++ {
 		tk.MustQuery("explain format='brief' select * from t force index(i) where id <= 1024 and c1 >= 0 and c1 <= 1024 and c2 in (2, 4, 6, 8) order by c1 limit 961").Check(kit.Rows(
@@ -678,4 +690,16 @@ func TestCopPaging(t *testing.T) {
 			"  └─Selection(Probe) 4.00 cop[tikv]  in(test.t.c2, 2, 4, 6, 8)",
 			"    └─TableRowIDScan 1024.00 cop[tikv] table:t keep order:false"))
 	}
+
+	// selection between limit and indexlookup, limit 961 should not go paging too
+	for i := 0; i < 10; i++ {
+		tk.MustQuery("explain format='brief' select * from t force index(i) where mod(id, 2) > 0 and id <= 1024 and c1 >= 0 and c1 <= 1024 and c2 in (2, 4, 6, 8) order by c1 limit 961").Check(kit.Rows(
+			"Limit 3.20 root  offset:0, count:961",
+			"└─Selection 2.56 root  gt(mod(test.t.id, 2), 0)",
+			"  └─IndexLookUp 3.20 root  ",
+			"    ├─Selection(Build) 819.20 cop[tikv]  le(test.t.id, 1024)",
+			"    │ └─IndexRangeScan 1024.00 cop[tikv] table:t, index:i(c1) range:[0,1024], keep order:true",
+			"    └─Selection(Probe) 3.20 cop[tikv]  in(test.t.c2, 2, 4, 6, 8)",
+			"      └─TableRowIDScan 819.20 cop[tikv] table:t keep order:false"))
+	}
 }

planner/core/task.go

@@ -925,6 +925,8 @@ func buildIndexLookUpTask(ctx sessionctx.Context, t *copTask) *rootTask {
 	if ctx.GetSessionVars().EnablePaging && t.expectCnt > 0 && t.expectCnt <= paging.Threshold {
 		p.Paging = true
 		pagingCst := calcPagingCost(ctx, t)
+		// prevent enlarging the cost because we take paging as a better plan,
+		// if the cost is enlarged, it'll be easier to go another plan.
 		idxCst = math.Min(idxCst, pagingCst)
 	}
 	newTask.cst += idxCst
@@ -1190,7 +1192,6 @@ func (p *PhysicalLimit) attach2Task(tasks ...task) task {
 			pushedDownLimit.SetSchema(pushedDownLimit.children[0].Schema())
 			pushedDownLimit.cost = cop.cost()
 		}
-		cop.expectCnt = p.Count
 		t = cop.convertToRootTask(p.ctx)
 		sunk = p.sinkIntoIndexLookUp(t)
 	} else if mpp, ok := t.(*mppTask); ok {