*: reduce NewRuntimeStatsColl() object allocation

distsql/distsql_test.go

@@ -167,7 +167,7 @@ func (s *testSuite) TestSelectResultRuntimeStats(c *C) {
 		rpcStat:          tikv.NewRegionRequestRuntimeStats(),
 	}
 	s2 := *s1
-	stmtStats := execdetails.NewRuntimeStatsColl()
+	stmtStats := execdetails.NewRuntimeStatsColl(nil)
 	stmtStats.RegisterStats(1, basic)
 	stmtStats.RegisterStats(1, s1)
 	stmtStats.RegisterStats(1, &s2)

distsql/select_result_test.go

@@ -33,7 +33,7 @@ func (s *testSuite) TestUpdateCopRuntimeStats(c *C) {
 	sr.rootPlanID = 1234
 	sr.updateCopRuntimeStats(context.Background(), &copr.CopRuntimeStats{ExecDetails: execdetails.ExecDetails{CalleeAddress: "a"}}, 0)
 
-	ctx.GetSessionVars().StmtCtx.RuntimeStatsColl = execdetails.NewRuntimeStatsColl()
+	ctx.GetSessionVars().StmtCtx.RuntimeStatsColl = execdetails.NewRuntimeStatsColl(nil)
 	t := uint64(1)
 	sr.selectResp = &tipb.SelectResponse{
 		ExecutionSummaries: []*tipb.ExecutorExecutionSummary{

executor/builder.go

@@ -970,7 +970,7 @@ func (b *executorBuilder) buildExplain(v *plannercore.Explain) Executor {
 	}
 	if v.Analyze {
 		if b.ctx.GetSessionVars().StmtCtx.RuntimeStatsColl == nil {
-			b.ctx.GetSessionVars().StmtCtx.RuntimeStatsColl = execdetails.NewRuntimeStatsColl()
+			b.ctx.GetSessionVars().StmtCtx.RuntimeStatsColl = execdetails.NewRuntimeStatsColl(nil)
 		}
 		explainExec.analyzeExec = b.build(v.TargetPlan)
 	}

executor/executor.go

@@ -1808,7 +1808,13 @@ func ResetContextOfStmt(ctx sessionctx.Context, s ast.StmtNode) (err error) {
 		sc.PrevAffectedRows = -1
 	}
 	if globalConfig.EnableCollectExecutionInfo {
-		sc.RuntimeStatsColl = execdetails.NewRuntimeStatsColl()
+		// In ExplainFor case, RuntimeStatsColl should not be reset for reuse,
+		// because ExplainFor need to display the last statement information.
+		reuseObj := vars.StmtCtx.RuntimeStatsColl
+		if _, ok := s.(*ast.ExplainForStmt); ok {
+			reuseObj = nil
+		}
+		sc.RuntimeStatsColl = execdetails.NewRuntimeStatsColl(reuseObj)
 	}
 
 	sc.TblInfo2UnionScan = make(map[*model.TableInfo]bool)

util/execdetails/execdetails.go

@@ -555,9 +555,23 @@ type RuntimeStatsColl struct {
 }
 
 // NewRuntimeStatsColl creates new executor collector.
-func NewRuntimeStatsColl() *RuntimeStatsColl {
-	return &RuntimeStatsColl{rootStats: make(map[int]*RootRuntimeStats),
-		copStats: make(map[int]*CopRuntimeStats)}
+// Reuse the object to reduce allocation when *RuntimeStatsColl is not nil.
+func NewRuntimeStatsColl(reuse *RuntimeStatsColl) *RuntimeStatsColl {
+	if reuse != nil {
+		// Reuse map is cheaper than create a new map object.
+		// Go compiler optimize this cleanup code pattern to a clearmap() function.
+		for k := range reuse.rootStats {
+			delete(reuse.rootStats, k)
+		}
+		for k := range reuse.copStats {
+			delete(reuse.copStats, k)
+		}
+		return reuse
+	}
+	return &RuntimeStatsColl{
+		rootStats: make(map[int]*RootRuntimeStats),
+		copStats:  make(map[int]*CopRuntimeStats),
+	}
 }
 
 // RegisterStats register execStat for a executor.

util/execdetails/execdetails_test.go

@@ -93,7 +93,7 @@ func mockExecutorExecutionSummaryForTiFlash(TimeProcessedNs, NumProducedRows, Nu
 }
 
 func TestCopRuntimeStats(t *testing.T) {
-	stats := NewRuntimeStatsColl()
+	stats := NewRuntimeStatsColl(nil)
 	tableScanID := 1
 	aggID := 2
 	tableReaderID := 3
@@ -156,7 +156,7 @@ func TestCopRuntimeStats(t *testing.T) {
 }
 
 func TestCopRuntimeStatsForTiFlash(t *testing.T) {
-	stats := NewRuntimeStatsColl()
+	stats := NewRuntimeStatsColl(nil)
 	tableScanID := 1
 	aggID := 2
 	tableReaderID := 3
@@ -261,7 +261,7 @@ func TestRootRuntimeStats(t *testing.T) {
 	basic1.Record(time.Second, 20)
 	basic2.Record(time.Second*2, 30)
 	pid := 1
-	stmtStats := NewRuntimeStatsColl()
+	stmtStats := NewRuntimeStatsColl(nil)
 	stmtStats.RegisterStats(pid, basic1)
 	stmtStats.RegisterStats(pid, basic2)
 	concurrency := &RuntimeStatsWithConcurrencyInfo{}