diff --git a/executor/executor.go b/executor/executor.go
index c832a59fb588f..9620a221ff63e 100644
--- a/executor/executor.go
+++ b/executor/executor.go
@@ -1921,7 +1921,7 @@ func ResetContextOfStmt(ctx sessionctx.Context, s ast.StmtNode) (err error) {
 	sc.OptimizerCETrace = nil
 
 	sc.SysdateIsNow = ctx.GetSessionVars().SysdateIsNow
-
+	sc.IsReadonlyStmt = plannercore.IsReadOnly(s, vars)
 	if _, ok := s.(*ast.AnalyzeTableStmt); ok {
 		sc.InitMemTracker(memory.LabelForAnalyzeMemory, -1)
 		sc.MemTracker.AttachTo(GlobalAnalyzeMemoryTracker)
diff --git a/planner/core/cache.go b/planner/core/cache.go
index 4b697b390fe10..28a51c2803613 100644
--- a/planner/core/cache.go
+++ b/planner/core/cache.go
@@ -156,16 +156,13 @@ func NewPlanCacheKey(sessionVars *variable.SessionVars, stmtText, stmtDB string,
 		lastUpdatedSchemaVersion: lastUpdatedSchemaVersion,
 		sqlMode:                  sessionVars.SQLMode,
 		timezoneOffset:           timezoneOffset,
-		isolationReadEngines:     make(map[kv.StoreType]struct{}),
+		isolationReadEngines:     sessionVars.GetAvailableIsolationReadEngines4Plan(),
 		selectLimit:              sessionVars.SelectLimit,
 		bindSQL:                  bindSQL,
 		inRestrictedSQL:          sessionVars.InRestrictedSQL,
 		restrictedReadOnly:       variable.RestrictedReadOnly.Load(),
 		TiDBSuperReadOnly:        variable.VarTiDBSuperReadOnly.Load(),
 	}
-	for k, v := range sessionVars.IsolationReadEngines {
-		key.isolationReadEngines[k] = v
-	}
 	return key, nil
 }
 
@@ -202,6 +199,9 @@ func (s FieldSlice) CheckTypesCompatibility4PC(tps []*types.FieldType) bool {
 }
 
 // PlanCacheValue stores the cached Statement and StmtNode.
+// Note: The variables' type shouldn't be put into the planCacheKey. Because for the decimal type, the different values for the decimal type may have different precise.
+// If we put it to the planCacheKey, it's hard to hit the plan cache. And if there exists lots of decimal values' statements, it will cost a lot of memory.
+// You can see the `CheckTypesCompatibility4PC` for more details about the type match.
 type PlanCacheValue struct {
 	Plan              Plan
 	OutPutNames       []*types.FieldName
diff --git a/planner/core/plan_cache.go b/planner/core/plan_cache.go
index c562be0218063..46dd35bf411db 100644
--- a/planner/core/plan_cache.go
+++ b/planner/core/plan_cache.go
@@ -88,8 +88,7 @@ func GetPlanFromSessionPlanCache(ctx context.Context, sctx sessionctx.Context, i
 		}
 	}
 
-	return generateNewPlan(ctx, sctx, is, preparedStmt, ignorePlanCache, cacheKey,
-		latestSchemaVersion, isBinProtocol, varsNum, binVarTypes, txtVarTypes, bindSQL)
+	return generateNewPlan(ctx, sctx, is, preparedStmt, ignorePlanCache, cacheKey, isBinProtocol, varsNum, binVarTypes, txtVarTypes)
 }
 
 // parseParamTypes get parameters' types in PREPARE statement
@@ -198,8 +197,8 @@ func getGeneralPlan(ctx context.Context, sctx sessionctx.Context, cacheKey kvcac
 // generateNewPlan call the optimizer to generate a new plan for current statement
 // and try to add it to cache
 func generateNewPlan(ctx context.Context, sctx sessionctx.Context, is infoschema.InfoSchema, preparedStmt *CachedPrepareStmt,
-	ignorePlanCache bool, cacheKey kvcache.Key, latestSchemaVersion int64, isBinProtocol bool, varsNum int, binVarTypes []byte,
-	txtVarTypes []*types.FieldType, bindSQL string) (Plan, []*types.FieldName, error) {
+	ignorePlanCache bool, cacheKey kvcache.Key, isBinProtocol bool, varsNum int, binVarTypes []byte,
+	txtVarTypes []*types.FieldType) (Plan, []*types.FieldName, error) {
 	prepared := preparedStmt.PreparedAst
 	sessVars := sctx.GetSessionVars()
 	stmtCtx := sessVars.StmtCtx
@@ -220,15 +219,6 @@ func generateNewPlan(ctx context.Context, sctx sessionctx.Context, is infoschema
 		stmtCtx.SkipPlanCache = true
 	}
 	if prepared.UseCache && !stmtCtx.SkipPlanCache && !ignorePlanCache {
-		// rebuild key to exclude kv.TiFlash when stmt is not read only
-		if _, isolationReadContainTiFlash := sessVars.IsolationReadEngines[kv.TiFlash]; isolationReadContainTiFlash && !IsReadOnly(stmt, sessVars) {
-			delete(sessVars.IsolationReadEngines, kv.TiFlash)
-			if cacheKey, err = NewPlanCacheKey(sessVars, preparedStmt.StmtText, preparedStmt.StmtDB,
-				prepared.SchemaVersion, latestSchemaVersion, bindSQL); err != nil {
-				return nil, nil, err
-			}
-			sessVars.IsolationReadEngines[kv.TiFlash] = struct{}{}
-		}
 		cached := NewPlanCacheValue(p, names, stmtCtx.TblInfo2UnionScan, isBinProtocol, binVarTypes, txtVarTypes)
 		preparedStmt.NormalizedPlan, preparedStmt.PlanDigest = NormalizePlan(p)
 		stmtCtx.SetPlan(p)
diff --git a/planner/core/planbuilder.go b/planner/core/planbuilder.go
index 4ca575b4b6a7f..49e3382e1b50b 100644
--- a/planner/core/planbuilder.go
+++ b/planner/core/planbuilder.go
@@ -1180,7 +1180,7 @@ func getPossibleAccessPaths(ctx sessionctx.Context, tableHints *tableHintInfo, i
 		}
 	}
 
-	_, isolationReadEnginesHasTiKV := ctx.GetSessionVars().GetIsolationReadEngines()[kv.TiKV]
+	isolationReadEnginesHasTiKV := ctx.GetSessionVars().ContainSpecialIsolationRead(kv.TiKV)
 	for i, hint := range indexHints {
 		if hint.HintScope != ast.HintForScan {
 			continue
@@ -1256,7 +1256,7 @@ func filterPathByIsolationRead(ctx sessionctx.Context, paths []*util.AccessPath,
 	if dbName.L == mysql.SystemDB {
 		return paths, nil
 	}
-	isolationReadEngines := ctx.GetSessionVars().GetIsolationReadEngines()
+	isolationReadEngines := ctx.GetSessionVars().GetAvailableIsolationReadEngines4Plan()
 	availableEngine := map[kv.StoreType]struct{}{}
 	var availableEngineStr string
 	for i := len(paths) - 1; i >= 0; i-- {
diff --git a/planner/optimize.go b/planner/optimize.go
index f1110b0ec6be2..916205d23aaab 100644
--- a/planner/optimize.go
+++ b/planner/optimize.go
@@ -88,15 +88,6 @@ func Optimize(ctx context.Context, sctx sessionctx.Context, node ast.Node, is in
 		}
 	}
 
-	// Because for write stmt, TiFlash has a different results when lock the data in point get plan. We ban the TiFlash
-	// engine in not read only stmt.
-	if _, isolationReadContainTiFlash := sessVars.IsolationReadEngines[kv.TiFlash]; isolationReadContainTiFlash && !IsReadOnly(node, sessVars) {
-		delete(sessVars.IsolationReadEngines, kv.TiFlash)
-		defer func() {
-			sessVars.IsolationReadEngines[kv.TiFlash] = struct{}{}
-		}()
-	}
-
 	tableHints := hint.ExtractTableHintsFromStmtNode(node, sctx)
 	originStmtHints, originStmtHintsOffs, warns := handleStmtHints(tableHints)
 	sessVars.StmtCtx.StmtHints = originStmtHints
@@ -112,7 +103,7 @@ func Optimize(ctx context.Context, sctx sessionctx.Context, node ast.Node, is in
 	}
 
 	txnManger := sessiontxn.GetTxnManager(sctx)
-	if _, isolationReadContainTiKV := sessVars.IsolationReadEngines[kv.TiKV]; isolationReadContainTiKV {
+	if sessVars.ContainSpecialIsolationRead(kv.TiKV) {
 		var fp core.Plan
 		if fpv, ok := sctx.Value(core.PointPlanKey).(core.PointPlanVal); ok {
 			// point plan is already tried in a multi-statement query.
diff --git a/server/conn.go b/server/conn.go
index b14ca98d67f46..f10fcdd8cbd34 100644
--- a/server/conn.go
+++ b/server/conn.go
@@ -1935,9 +1935,9 @@ func (cc *clientConn) handleQuery(ctx context.Context, sql string) (err error) {
 			// When the TiFlash server seems down, we append a warning to remind the user to check the status of the TiFlash
 			// server and fallback to TiKV.
 			warns := append(parserWarns, stmtctx.SQLWarn{Level: stmtctx.WarnLevelError, Err: err})
-			delete(cc.ctx.GetSessionVars().IsolationReadEngines, kv.TiFlash)
+			cc.ctx.GetSessionVars().StmtCtx.InTiFlashFallBack2TiKV = true
 			_, err = cc.handleStmt(ctx, stmt, warns, i == len(stmts)-1)
-			cc.ctx.GetSessionVars().IsolationReadEngines[kv.TiFlash] = struct{}{}
+			cc.ctx.GetSessionVars().StmtCtx.InTiFlashFallBack2TiKV = false
 			if err != nil {
 				break
 			}
diff --git a/server/conn_stmt.go b/server/conn_stmt.go
index e1b7ff416b2c3..b7e82544ca54c 100644
--- a/server/conn_stmt.go
+++ b/server/conn_stmt.go
@@ -217,11 +217,9 @@ func (cc *clientConn) handleStmtExecute(ctx context.Context, data []byte) (err e
 		// When the TiFlash server seems down, we append a warning to remind the user to check the status of the TiFlash
 		// server and fallback to TiKV.
 		prevErr := err
-		delete(cc.ctx.GetSessionVars().IsolationReadEngines, kv.TiFlash)
-		defer func() {
-			cc.ctx.GetSessionVars().IsolationReadEngines[kv.TiFlash] = struct{}{}
-		}()
+		cc.ctx.GetSessionVars().StmtCtx.InTiFlashFallBack2TiKV = true
 		_, err = cc.executePreparedStmtAndWriteResult(ctx, stmt, args, useCursor)
+		cc.ctx.GetSessionVars().StmtCtx.InTiFlashFallBack2TiKV = false
 		// We append warning after the retry because `ResetContextOfStmt` may be called during the retry, which clears warnings.
 		cc.ctx.GetSessionVars().StmtCtx.AppendError(prevErr)
 	}
diff --git a/session/session.go b/session/session.go
index d6c462e1f0bab..81448c7a16ca8 100644
--- a/session/session.go
+++ b/session/session.go
@@ -363,7 +363,7 @@ func (s *session) cleanRetryInfo() {
 	for i, stmtID := range retryInfo.DroppedPreparedStmtIDs {
 		if planCacheEnabled {
 			if i > 0 && preparedAst != nil {
-				plannercore.SetPstmtIDSchemaVersion(cacheKey, stmtText, preparedAst.SchemaVersion, s.sessionVars.IsolationReadEngines)
+				plannercore.SetPstmtIDSchemaVersion(cacheKey, stmtText, preparedAst.SchemaVersion, s.sessionVars.GetAvailableIsolationReadEngines4Plan())
 			}
 			if !s.sessionVars.IgnorePreparedCacheCloseStmt { // keep the plan in cache
 				s.PreparedPlanCache().Delete(cacheKey)
diff --git a/sessionctx/stmtctx/stmtctx.go b/sessionctx/stmtctx/stmtctx.go
index a13b80a78ae69..079ec827a1c70 100644
--- a/sessionctx/stmtctx/stmtctx.go
+++ b/sessionctx/stmtctx/stmtctx.go
@@ -139,6 +139,8 @@ type StatementContext struct {
 	SkipASCIICheck         bool
 	SkipUTF8MB4Check       bool
 	MultiSchemaInfo        *model.MultiSchemaInfo
+	IsReadonlyStmt         bool
+	InTiFlashFallBack2TiKV bool
 	// If the select statement was like 'select * from t as of timestamp ...' or in a stale read transaction
 	// or is affected by the tidb_read_staleness session variable, then the statement will be makred as isStaleness
 	// in stmtCtx
diff --git a/sessionctx/variable/session.go b/sessionctx/variable/session.go
index aaa64d969450a..d5707de609320 100644
--- a/sessionctx/variable/session.go
+++ b/sessionctx/variable/session.go
@@ -979,8 +979,8 @@ type SessionVars struct {
 	// this variable only take effect when `tidb_follower_read` = 'closest-adaptive'
 	ReplicaClosestReadThreshold int64
 
-	// IsolationReadEngines is used to isolation read, tidb only read from the stores whose engine type is in the engines.
-	IsolationReadEngines map[kv.StoreType]struct{}
+	// isolationReadEngines is used to isolation read, tidb only read from the stores whose engine type is in the engines.
+	isolationReadEngines map[kv.StoreType]struct{}
 
 	PlannerSelectBlockAsName []ast.HintTable
 
@@ -1410,7 +1410,7 @@ func NewSessionVars() *SessionVars {
 		UsePlanBaselines:            DefTiDBUsePlanBaselines,
 		EvolvePlanBaselines:         DefTiDBEvolvePlanBaselines,
 		EnableExtendedStats:         false,
-		IsolationReadEngines:        make(map[kv.StoreType]struct{}),
+		isolationReadEngines:        make(map[kv.StoreType]struct{}),
 		LockWaitTimeout:             DefInnodbLockWaitTimeout * 1000,
 		MetricSchemaStep:            DefTiDBMetricSchemaStep,
 		MetricSchemaRangeDuration:   DefTiDBMetricSchemaRangeDuration,
@@ -1485,11 +1485,11 @@ func NewSessionVars() *SessionVars {
 	for _, engine := range config.GetGlobalConfig().IsolationRead.Engines {
 		switch engine {
 		case kv.TiFlash.Name():
-			vars.IsolationReadEngines[kv.TiFlash] = struct{}{}
+			vars.isolationReadEngines[kv.TiFlash] = struct{}{}
 		case kv.TiKV.Name():
-			vars.IsolationReadEngines[kv.TiKV] = struct{}{}
+			vars.isolationReadEngines[kv.TiKV] = struct{}{}
 		case kv.TiDB.Name():
-			vars.IsolationReadEngines[kv.TiDB] = struct{}{}
+			vars.isolationReadEngines[kv.TiDB] = struct{}{}
 		}
 	}
 	if !EnableLocalTxn.Load() {
@@ -1578,9 +1578,33 @@ func (s *SessionVars) GetSplitRegionTimeout() time.Duration {
 	return time.Duration(s.WaitSplitRegionTimeout) * time.Second
 }
 
+// ContainSpecialIsolationRead checks whether we can access the special read engines.
+func (s *SessionVars) ContainSpecialIsolationRead(engineType kv.StoreType) bool {
+	_, ok := s.isolationReadEngines[engineType]
+	return ok
+}
+
 // GetIsolationReadEngines gets isolation read engines.
+// We copy the s.isolationReadEngines and return the copied one.
+// So the change in the outside will not affect the origin value.
 func (s *SessionVars) GetIsolationReadEngines() map[kv.StoreType]struct{} {
-	return s.IsolationReadEngines
+	readEngines := make(map[kv.StoreType]struct{})
+	for isolationReadEngine := range s.isolationReadEngines {
+		readEngines[isolationReadEngine] = struct{}{}
+	}
+	return readEngines
+}
+
+// GetAvailableIsolationReadEngines4Plan gets the available read engines for the current statement.
+// We should use this function to get the isolation read engines in the optimize phase.
+// For example, for write stmt, TiFlash has a different results when lock the data in point get plan.
+// So we ban the TiFlash engine in not read only stmt.
+func (s *SessionVars) GetAvailableIsolationReadEngines4Plan() map[kv.StoreType]struct{} {
+	availableReadEngines := s.GetIsolationReadEngines()
+	if _, isolationReadContainTiFlash := availableReadEngines[kv.TiFlash]; isolationReadContainTiFlash && (s.StmtCtx.IsReadonlyStmt || s.StmtCtx.InTiFlashFallBack2TiKV) {
+		delete(availableReadEngines, kv.TiFlash)
+	}
+	return availableReadEngines
 }
 
 // CleanBuffers cleans the temporary bufs
diff --git a/sessionctx/variable/session_test.go b/sessionctx/variable/session_test.go
index b94a261edc069..cc084ee55138a 100644
--- a/sessionctx/variable/session_test.go
+++ b/sessionctx/variable/session_test.go
@@ -284,11 +284,11 @@ func TestIsolationRead(t *testing.T) {
 		conf.IsolationRead.Engines = []string{"tiflash", "tidb"}
 	})
 	sessVars := variable.NewSessionVars()
-	_, ok := sessVars.IsolationReadEngines[kv.TiDB]
+	ok := sessVars.ContainSpecialIsolationRead(kv.TiDB)
 	require.True(t, ok)
-	_, ok = sessVars.IsolationReadEngines[kv.TiKV]
+	ok = sessVars.ContainSpecialIsolationRead(kv.TiKV)
 	require.False(t, ok)
-	_, ok = sessVars.IsolationReadEngines[kv.TiFlash]
+	ok = sessVars.ContainSpecialIsolationRead(kv.TiFlash)
 	require.True(t, ok)
 }
 
diff --git a/sessionctx/variable/sysvar.go b/sessionctx/variable/sysvar.go
index 0e0b45fd7d018..7bd1adca1e627 100644
--- a/sessionctx/variable/sysvar.go
+++ b/sessionctx/variable/sysvar.go
@@ -264,15 +264,15 @@ var defaultSysVars = []*SysVar{
 		}
 		return formatVal, nil
 	}, SetSession: func(s *SessionVars, val string) error {
-		s.IsolationReadEngines = make(map[kv.StoreType]struct{})
+		s.isolationReadEngines = make(map[kv.StoreType]struct{})
 		for _, engine := range strings.Split(val, ",") {
 			switch engine {
 			case kv.TiKV.Name():
-				s.IsolationReadEngines[kv.TiKV] = struct{}{}
+				s.isolationReadEngines[kv.TiKV] = struct{}{}
 			case kv.TiFlash.Name():
-				s.IsolationReadEngines[kv.TiFlash] = struct{}{}
+				s.isolationReadEngines[kv.TiFlash] = struct{}{}
 			case kv.TiDB.Name():
-				s.IsolationReadEngines[kv.TiDB] = struct{}{}
+				s.isolationReadEngines[kv.TiDB] = struct{}{}
 			}
 		}
 		return nil