planner: add a switch control if prepare stmt with param limit can be cached

executor/seqtest/prepared_test.go

@@ -806,3 +806,63 @@ func TestIssue38323(t *testing.T) {
 	tk.MustExec("set @a = 1;")
 	tk.MustQuery("execute stmt using @a, @a").Check(tk.MustQuery("explain select * from t where 1 = id and 1 = k group by id, k").Rows())
 }
+
+func TestSetPlanCacheLimitSwitch(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+
+	tk.MustQuery("select @@session.tidb_enable_plan_cache_for_param_limit").Check(testkit.Rows("1"))
+	tk.MustQuery("select @@global.tidb_enable_plan_cache_for_param_limit").Check(testkit.Rows("1"))
+
+	tk.MustExec("set @@session.tidb_enable_plan_cache_for_param_limit = OFF;")
+	tk.MustQuery("select @@session.tidb_enable_plan_cache_for_param_limit").Check(testkit.Rows("0"))
+
+	tk.MustExec("set @@session.tidb_enable_plan_cache_for_param_limit = 1;")
+	tk.MustQuery("select @@session.tidb_enable_plan_cache_for_param_limit").Check(testkit.Rows("1"))
+
+	tk.MustExec("set @@global.tidb_enable_plan_cache_for_param_limit = off;")
+	tk.MustQuery("select @@global.tidb_enable_plan_cache_for_param_limit").Check(testkit.Rows("0"))
+
+	tk.MustExec("set @@global.tidb_enable_plan_cache_for_param_limit = ON;")
+	tk.MustQuery("select @@global.tidb_enable_plan_cache_for_param_limit").Check(testkit.Rows("1"))
+}
+
+func TestPlanCacheLimitSwitchEffective(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int, key(a))")
+
+	checkIfCached := func(res string) {
+		tk.MustExec("set @a = 1")
+		tk.MustExec("execute stmt using @a")
+		tk.MustExec("execute stmt using @a")
+		tk.MustQuery("select @@last_plan_from_cache").Check(testkit.Rows(res))
+	}
+
+	// before prepare
+	tk.MustExec("set @@session.tidb_enable_plan_cache_for_param_limit = OFF")
+	tk.MustExec("prepare stmt from 'select * from t limit ?'")
+	tk.MustQuery("show warnings").Check(testkit.Rows("Warning 1105 skip plan-cache: query has 'limit ?' is un-cacheable"))
+	checkIfCached("0")
+	tk.MustExec("deallocate prepare stmt")
+
+	// after prepare
+	tk.MustExec("set @@session.tidb_enable_plan_cache_for_param_limit = ON")
+	tk.MustExec("prepare stmt from 'select * from t limit ?'")
+	tk.MustExec("set @@session.tidb_enable_plan_cache_for_param_limit = OFF")
+	checkIfCached("0")
+	tk.MustExec("execute stmt using @a")
+	tk.MustQuery("show warnings").Check(testkit.Rows("Warning 1105 skip plan-cache: the switch 'tidb_enable_plan_cache_for_param_limit' is off"))
+	tk.MustExec("deallocate prepare stmt")
+
+	// after execute
+	tk.MustExec("set @@session.tidb_enable_plan_cache_for_param_limit = ON")
+	tk.MustExec("prepare stmt from 'select * from t limit ?'")
+	checkIfCached("1")
+	tk.MustExec("set @@session.tidb_enable_plan_cache_for_param_limit = OFF")
+	checkIfCached("0")
+	tk.MustExec("deallocate prepare stmt")
+}

planner/core/plan_cache.go

@@ -287,7 +287,7 @@ func generateNewPlan(ctx context.Context, sctx sessionctx.Context, isNonPrepared
 
 	// check whether this plan is cacheable.
 	if stmtCtx.UseCache {
-		checkPlanCacheability(sctx, p, len(paramTypes))
+		checkPlanCacheability(sctx, p, len(paramTypes), len(limitParams))
 	}
 
 	// put this plan into the plan cache.
@@ -312,7 +312,7 @@ func generateNewPlan(ctx context.Context, sctx sessionctx.Context, isNonPrepared
 }
 
 // checkPlanCacheability checks whether this plan is cacheable and set to skip plan cache if it's uncacheable.
-func checkPlanCacheability(sctx sessionctx.Context, p Plan, paramNum int) {
+func checkPlanCacheability(sctx sessionctx.Context, p Plan, paramNum int, limitParamNum int) {
 	stmtCtx := sctx.GetSessionVars().StmtCtx
 	var pp PhysicalPlan
 	switch x := p.(type) {
@@ -347,6 +347,11 @@ func checkPlanCacheability(sctx sessionctx.Context, p Plan, paramNum int) {
 		stmtCtx.SetSkipPlanCache(errors.New("skip plan-cache: the plan with IndexMerge accessing Multi-Valued Index is un-cacheable"))
 		return
 	}
+
+	// before cache the param limit plan, check switch
+	if limitParamNum != 0 && !sctx.GetSessionVars().EnablePlanCacheForParamLimit {
+		stmtCtx.SetSkipPlanCache(errors.New("skip plan-cache: the switch 'tidb_enable_plan_cache_for_param_limit' is off"))
+	}
 }
 
 // RebuildPlan4CachedPlan will rebuild this plan under current user parameters.

planner/core/plan_cache_lru.go

@@ -258,14 +258,22 @@ func (l *LRUPlanCache) memoryControl() {
 func (l *LRUPlanCache) pickFromBucket(bucket map[*list.Element]struct{}, matchOpts *planCacheMatchOpts) (*list.Element, bool) {
 	for k := range bucket {
 		plan := k.Value.(*planCacheEntry).PlanValue.(*PlanCacheValue)
+		// check param types' compatibility
 		ok1 := plan.matchOpts.paramTypes.CheckTypesCompatibility4PC(matchOpts.paramTypes)
 		if !ok1 {
 			continue
 		}
+
+		// check limit offset and key if equal and check switch if enabled
 		ok2 := checkUint64SliceIfEqual(plan.matchOpts.limitOffsetAndCount, matchOpts.limitOffsetAndCount)
-		if ok2 {
-			return k, true
+		if !ok2 {
+			continue
+		}
+		if len(plan.matchOpts.limitOffsetAndCount) > 0 && !l.sctx.GetSessionVars().EnablePlanCacheForParamLimit {
+			// offset and key slice matched, but it is a plan with param limit and the switch is disabled
+			continue
 		}
+		return k, true
 	}
 	return nil, false
 }

planner/core/plan_cache_lru_test.go

@@ -47,11 +47,13 @@ func randomPlanCacheValue(types []*types.FieldType) *PlanCacheValue {
 
 func TestLRUPCPut(t *testing.T) {
 	// test initialize
-	lruA := NewLRUPlanCache(0, 0, 0, MockContext())
+	mockCtx := MockContext()
+	mockCtx.GetSessionVars().EnablePlanCacheForParamLimit = true
+	lruA := NewLRUPlanCache(0, 0, 0, mockCtx)
 	require.Equal(t, lruA.capacity, uint(100))
 
 	maxMemDroppedKv := make(map[kvcache.Key]kvcache.Value)
-	lru := NewLRUPlanCache(3, 0, 0, MockContext())
+	lru := NewLRUPlanCache(3, 0, 0, mockCtx)
 	lru.onEvict = func(key kvcache.Key, value kvcache.Value) {
 		maxMemDroppedKv[key] = value
 	}
@@ -131,7 +133,9 @@ func TestLRUPCPut(t *testing.T) {
 }
 
 func TestLRUPCGet(t *testing.T) {
-	lru := NewLRUPlanCache(3, 0, 0, MockContext())
+	mockCtx := MockContext()
+	mockCtx.GetSessionVars().EnablePlanCacheForParamLimit = true
+	lru := NewLRUPlanCache(3, 0, 0, mockCtx)
 
 	keys := make([]*planCacheKey, 5)
 	vals := make([]*PlanCacheValue, 5)
@@ -185,7 +189,9 @@ func TestLRUPCGet(t *testing.T) {
 }
 
 func TestLRUPCDelete(t *testing.T) {
-	lru := NewLRUPlanCache(3, 0, 0, MockContext())
+	mockCtx := MockContext()
+	mockCtx.GetSessionVars().EnablePlanCacheForParamLimit = true
+	lru := NewLRUPlanCache(3, 0, 0, mockCtx)
 
 	keys := make([]*planCacheKey, 3)
 	vals := make([]*PlanCacheValue, 3)

planner/core/plan_cacheable_checker.go

@@ -135,22 +135,24 @@ func (checker *cacheableChecker) Enter(in ast.Node) (out ast.Node, skipChildren
 				return in, true
 			}
 		}
-	// todo: these comment is used to add switch in the later pr
-	//case *ast.Limit:
-	//	if node.Count != nil {
-	//		if _, isParamMarker := node.Count.(*driver.ParamMarkerExpr); isParamMarker {
-	//			checker.cacheable = false
-	//			checker.reason = "query has 'limit ?' is un-cacheable"
-	//			return in, true
-	//		}
-	//	}
-	//	if node.Offset != nil {
-	//		if _, isParamMarker := node.Offset.(*driver.ParamMarkerExpr); isParamMarker {
-	//			checker.cacheable = false
-	//			checker.reason = "query has 'limit ?, 10' is un-cacheable"
-	//			return in, true
-	//		}
-	//	}
+	case *ast.Limit:
+		if checker.sctx.GetSessionVars().EnablePlanCacheForParamLimit {
+			return in, false
+		}
+		if node.Count != nil {
+			if _, isParamMarker := node.Count.(*driver.ParamMarkerExpr); isParamMarker {
+				checker.cacheable = false
+				checker.reason = "query has 'limit ?' is un-cacheable"
+				return in, true
+			}
+		}
+		if node.Offset != nil {
+			if _, isParamMarker := node.Offset.(*driver.ParamMarkerExpr); isParamMarker {
+				checker.cacheable = false
+				checker.reason = "query has 'limit ?, 10' is un-cacheable"
+				return in, true
+			}
+		}
 	case *ast.FrameBound:
 		if _, ok := node.Expr.(*driver.ParamMarkerExpr); ok {
 			checker.cacheable = false

planner/core/plan_cacheable_checker_test.go

@@ -26,11 +26,14 @@ import (
 	"github.com/pingcap/tidb/planner/core"
 	"github.com/pingcap/tidb/testkit"
 	driver "github.com/pingcap/tidb/types/parser_driver"
+	"github.com/pingcap/tidb/util/mock"
 	"github.com/stretchr/testify/require"
 )
 
 func TestCacheable(t *testing.T) {
 	store := testkit.CreateMockStore(t)
+	mockCtx := mock.NewContext()
+	mockCtx.GetSessionVars().EnablePlanCacheForParamLimit = true
 
 	tk := testkit.NewTestKit(t, store)
 
@@ -87,7 +90,8 @@ func TestCacheable(t *testing.T) {
 		TableRefs: tableRefsClause,
 		Limit:     limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ := core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	limitStmt = &ast.Limit{
 		Offset: &driver.ParamMarkerExpr{},
@@ -96,14 +100,16 @@ func TestCacheable(t *testing.T) {
 		TableRefs: tableRefsClause,
 		Limit:     limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ = core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	limitStmt = &ast.Limit{}
 	stmt = &ast.DeleteStmt{
 		TableRefs: tableRefsClause,
 		Limit:     limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ = core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	stmt.(*ast.DeleteStmt).TableHints = append(stmt.(*ast.DeleteStmt).TableHints, &ast.TableOptimizerHint{
 		HintName: model.NewCIStr(core.HintIgnorePlanCache),
@@ -139,7 +145,8 @@ func TestCacheable(t *testing.T) {
 		TableRefs: tableRefsClause,
 		Limit:     limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ = core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	limitStmt = &ast.Limit{
 		Offset: &driver.ParamMarkerExpr{},
@@ -148,14 +155,16 @@ func TestCacheable(t *testing.T) {
 		TableRefs: tableRefsClause,
 		Limit:     limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ = core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	limitStmt = &ast.Limit{}
 	stmt = &ast.UpdateStmt{
 		TableRefs: tableRefsClause,
 		Limit:     limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ = core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	stmt.(*ast.UpdateStmt).TableHints = append(stmt.(*ast.UpdateStmt).TableHints, &ast.TableOptimizerHint{
 		HintName: model.NewCIStr(core.HintIgnorePlanCache),
@@ -188,21 +197,24 @@ func TestCacheable(t *testing.T) {
 	stmt = &ast.SelectStmt{
 		Limit: limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ = core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	limitStmt = &ast.Limit{
 		Offset: &driver.ParamMarkerExpr{},
 	}
 	stmt = &ast.SelectStmt{
 		Limit: limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ = core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	limitStmt = &ast.Limit{}
 	stmt = &ast.SelectStmt{
 		Limit: limitStmt,
 	}
-	require.True(t, core.Cacheable(stmt, is))
+	c, _ = core.CacheableWithCtx(mockCtx, stmt, is)
+	require.True(t, c)
 
 	paramExpr := &driver.ParamMarkerExpr{}
 	orderByClause := &ast.OrderByClause{Items: []*ast.ByItem{{Expr: paramExpr}}}

sessionctx/variable/session.go

@@ -1253,6 +1253,9 @@ type SessionVars struct {
 	// PreparedPlanCacheMonitor indicates whether to enable prepared plan cache monitor.
 	EnablePreparedPlanCacheMemoryMonitor bool
 
+	// EnablePlanCacheForParamLimit controls whether the prepare statement with parameterized limit can be cached
+	EnablePlanCacheForParamLimit bool
+
 	// EnableNonPreparedPlanCache indicates whether to enable non-prepared plan cache.
 	EnableNonPreparedPlanCache bool
 

sessionctx/variable/sysvar.go

@@ -2267,6 +2267,10 @@ var defaultSysVars = []*SysVar{
 		s.PessimisticTransactionAggressiveLocking = TiDBOptOn(val)
 		return nil
 	}},
+	{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnablePlanCacheForParamLimit, Value: BoolToOnOff(DefTiDBEnablePlanCacheForParamLimit), Type: TypeBool, SetSession: func(s *SessionVars, val string) error {
+		s.EnablePlanCacheForParamLimit = TiDBOptOn(val)
+		return nil
+	}},
 }
 
 // FeedbackProbability points to the FeedbackProbability in statistics package.

sessionctx/variable/tidb_vars.go

@@ -796,6 +796,9 @@ const (
 	// TiDBPessimisticTransactionAggressiveLocking controls whether aggressive locking for pessimistic transaction
 	// is enabled.
 	TiDBPessimisticTransactionAggressiveLocking = "tidb_pessimistic_txn_aggressive_locking"
+
+	// TiDBEnablePlanCacheForParamLimit controls whether prepare statement with parameterized limit can be cached
+	TiDBEnablePlanCacheForParamLimit = "tidb_enable_plan_cache_for_param_limit"
 )
 
 // TiDB vars that have only global scope
@@ -1167,6 +1170,7 @@ const (
 	DefTiDBTTLDeleteWorkerCount                            = 4
 	DefTiDBEnableResourceControl                           = false
 	DefTiDBPessimisticTransactionAggressiveLocking         = false
+	DefTiDBEnablePlanCacheForParamLimit                    = true
 )
 
 // Process global variables.