sessionctx: enable IndexMerge by default

cmd/explaintest/r/explain_indexmerge.result

@@ -6,19 +6,23 @@ create index td on t (d);
 load stats 's/explain_indexmerge_stats_t.json';
 explain format = 'brief' select * from t where a < 50 or b < 50;
 id	estRows	task	access object	operator info
-TableReader	98.00	root		data:Selection
-└─Selection	98.00	cop[tikv]		or(lt(test.t.a, 50), lt(test.t.b, 50))
-  └─TableFullScan	5000000.00	cop[tikv]	table:t	keep order:false
+IndexMerge	98.00	root		
+├─TableRangeScan(Build)	49.00	cop[tikv]	table:t	range:[-inf,50), keep order:false
+├─IndexRangeScan(Build)	49.00	cop[tikv]	table:t, index:tb(b)	range:[-inf,50), keep order:false
+└─TableRowIDScan(Probe)	98.00	cop[tikv]	table:t	keep order:false
 explain format = 'brief' select * from t where (a < 50 or b < 50) and f > 100;
 id	estRows	task	access object	operator info
-TableReader	98.00	root		data:Selection
-└─Selection	98.00	cop[tikv]		gt(test.t.f, 100), or(lt(test.t.a, 50), lt(test.t.b, 50))
-  └─TableFullScan	5000000.00	cop[tikv]	table:t	keep order:false
+IndexMerge	98.00	root		
+├─TableRangeScan(Build)	49.00	cop[tikv]	table:t	range:[-inf,50), keep order:false
+├─IndexRangeScan(Build)	49.00	cop[tikv]	table:t, index:tb(b)	range:[-inf,50), keep order:false
+└─Selection(Probe)	98.00	cop[tikv]		gt(test.t.f, 100)
+  └─TableRowIDScan	98.00	cop[tikv]	table:t	keep order:false
 explain format = 'brief' select * from t where b < 50 or c < 50;
 id	estRows	task	access object	operator info
-TableReader	98.00	root		data:Selection
-└─Selection	98.00	cop[tikv]		or(lt(test.t.b, 50), lt(test.t.c, 50))
-  └─TableFullScan	5000000.00	cop[tikv]	table:t	keep order:false
+IndexMerge	98.00	root		
+├─IndexRangeScan(Build)	49.00	cop[tikv]	table:t, index:tb(b)	range:[-inf,50), keep order:false
+├─IndexRangeScan(Build)	49.00	cop[tikv]	table:t, index:tc(c)	range:[-inf,50), keep order:false
+└─TableRowIDScan(Probe)	98.00	cop[tikv]	table:t	keep order:false
 set session tidb_enable_index_merge = on;
 explain format = 'brief' select * from t where a < 50 or b < 50;
 id	estRows	task	access object	operator info

planner/core/integration_test.go

@@ -2203,6 +2203,10 @@ func (s *testIntegrationSuite) TestOptimizeHintOnPartitionTable(c *C) {
 					partition p1 values less than(11),
 					partition p2 values less than(16));`)
 	tk.MustExec(`insert into t values (1,1,"1"), (2,2,"2"), (8,8,"8"), (11,11,"11"), (15,15,"15")`)
+	tk.MustExec("set @@tidb_enable_index_merge = off")
+	defer func() {
+		tk.MustExec("set @@tidb_enable_index_merge = on")
+	}()
 
 	// Create virtual tiflash replica info.
 	dom := domain.GetDomain(tk.Se)

planner/core/testdata/plan_suite_out.json

@@ -286,9 +286,9 @@
       },
       {
         "SQL": "select /*+ USE_INDEX_MERGE(t1, c_d_e, f_g) */ * from t where c < 1 or f > 2",
-        "Best": "TableReader(Table(t)->Sel([or(lt(test.t.c, 1), gt(test.t.f, 2))]))",
+        "Best": "IndexMergeReader(PartialPlans->[Index(t.c_d_e)[[-inf,1)], Index(t.f)[(2,+inf]]], TablePlan->Table(t))",
         "HasWarn": true,
-        "Hints": "use_index(@`sel_1` `test`.`t` )"
+        "Hints": "use_index_merge(@`sel_1` `t` `c_d_e`, `f`)"
       },
       {
         "SQL": "select /*+ NO_INDEX_MERGE(), USE_INDEX_MERGE(t, primary, f_g, c_d_e) */ * from t where a < 1 or f > 2",
@@ -304,15 +304,15 @@
       },
       {
         "SQL": "select /*+ USE_INDEX_MERGE(db2.t) */ * from t where c < 1 or f > 2",
-        "Best": "TableReader(Table(t)->Sel([or(lt(test.t.c, 1), gt(test.t.f, 2))]))",
+        "Best": "IndexMergeReader(PartialPlans->[Index(t.c_d_e)[[-inf,1)], Index(t.f)[(2,+inf]]], TablePlan->Table(t))",
         "HasWarn": true,
-        "Hints": "use_index(@`sel_1` `test`.`t` )"
+        "Hints": "use_index_merge(@`sel_1` `t` `c_d_e`, `f`)"
       },
       {
         "SQL": "select /*+ USE_INDEX_MERGE(db2.t, c_d_e, f_g) */ * from t where c < 1 or f > 2",
-        "Best": "TableReader(Table(t)->Sel([or(lt(test.t.c, 1), gt(test.t.f, 2))]))",
+        "Best": "IndexMergeReader(PartialPlans->[Index(t.c_d_e)[[-inf,1)], Index(t.f)[(2,+inf]]], TablePlan->Table(t))",
         "HasWarn": true,
-        "Hints": "use_index(@`sel_1` `test`.`t` )"
+        "Hints": "use_index_merge(@`sel_1` `t` `c_d_e`, `f`)"
       }
     ]
   },

session/bootstrap.go

@@ -541,11 +541,14 @@ const (
 	// version80 fixes the issue https://github.com/pingcap/tidb/issues/25422.
 	// If the TiDB upgrading from the 4.x to a newer version, we keep the tidb_analyze_version to 1.
 	version80 = 80
+	// version81 insert "tidb_enable_index_merge|off" to mysql.GLOBAL_VARIABLES if there is no tidb_enable_index_merge.
+	// This will only happens when we upgrade a cluster before 4.0.0 to 4.0.0+.
+	version81 = 81
 )
 
 // currentBootstrapVersion is defined as a variable, so we can modify its value for testing.
 // please make sure this is the largest version
-var currentBootstrapVersion int64 = version80
+var currentBootstrapVersion int64 = version81
 
 var (
 	bootstrapVersion = []func(Session, int64){
@@ -629,6 +632,7 @@ var (
 		upgradeToVer78,
 		upgradeToVer79,
 		upgradeToVer80,
+		upgradeToVer81,
 	}
 )
 
@@ -1655,6 +1659,29 @@ func upgradeToVer80(s Session, ver int64) {
 		mysql.SystemDB, mysql.GlobalVariablesTable, variable.TiDBAnalyzeVersion, 1)
 }
 
+// For users that upgrade TiDB from a pre-4.0 version, we want to disable index merge by default.
+// This helps minimize query plan regressions.
+func upgradeToVer81(s Session, ver int64) {
+	if ver >= version81 {
+		return
+	}
+	// Check if tidb_enable_index_merge exists in mysql.GLOBAL_VARIABLES.
+	// If not, insert "tidb_enable_index_merge | off".
+	ctx := context.Background()
+	rs, err := s.ExecuteInternal(ctx, "SELECT VARIABLE_VALUE FROM %n.%n WHERE VARIABLE_NAME=%?;",
+		mysql.SystemDB, mysql.GlobalVariablesTable, variable.TiDBEnableIndexMerge)
+	terror.MustNil(err)
+	req := rs.NewChunk(nil)
+	err = rs.Next(ctx, req)
+	terror.MustNil(err)
+	if req.NumRows() != 0 {
+		return
+	}
+
+	mustExecute(s, "INSERT HIGH_PRIORITY IGNORE INTO %n.%n VALUES (%?, %?);",
+		mysql.SystemDB, mysql.GlobalVariablesTable, variable.TiDBEnableIndexMerge, variable.Off)
+}
+
 func writeOOMAction(s Session) {
 	comment := "oom-action is `log` by default in v3.0.x, `cancel` by default in v4.0.11+"
 	mustExecute(s, `INSERT HIGH_PRIORITY INTO %n.%n VALUES (%?, %?, %?) ON DUPLICATE KEY UPDATE VARIABLE_VALUE= %?`,

session/bootstrap_test.go

@@ -928,3 +928,144 @@ func TestAnalyzeVersionUpgradeFrom300To500(t *testing.T) {
 	require.Equal(t, 1, row.Len())
 	require.Equal(t, "1", row.GetString(0))
 }
+
+func TestIndexMergeInNewCluster(t *testing.T) {
+	store, err := mockstore.NewMockStore()
+	require.NoError(t, err)
+	// Indicates we are in a new cluster.
+	require.Equal(t, int64(notBootstrapped), getStoreBootstrapVersion(store))
+	dom, err := BootstrapSession(store)
+	require.NoError(t, err)
+	defer func() { require.NoError(t, store.Close()) }()
+	defer dom.Close()
+	se := createSessionAndSetID(t, store)
+
+	// In a new created cluster(above 5.4+), tidb_enable_index_merge is 1 by default.
+	mustExec(t, se, "use test;")
+	r := mustExec(t, se, "select @@tidb_enable_index_merge;")
+	require.NotNil(t, r)
+
+	ctx := context.Background()
+	chk := r.NewChunk(nil)
+	err = r.Next(ctx, chk)
+	require.NoError(t, err)
+	require.Equal(t, 1, chk.NumRows())
+	row := chk.GetRow(0)
+	require.Equal(t, 1, row.Len())
+	require.Equal(t, int64(1), row.GetInt64(0))
+}
+
+func TestIndexMergeUpgradeFrom300To540(t *testing.T) {
+	ctx := context.Background()
+	store, _ := createStoreAndBootstrap(t)
+	defer func() { require.NoError(t, store.Close()) }()
+
+	// Upgrade from 3.0.0 to 5.4+.
+	ver300 := 33
+	seV3 := createSessionAndSetID(t, store)
+	txn, err := store.Begin()
+	require.NoError(t, err)
+	m := meta.NewMeta(txn)
+	err = m.FinishBootstrap(int64(ver300))
+	require.NoError(t, err)
+	err = txn.Commit(context.Background())
+	require.NoError(t, err)
+	mustExec(t, seV3, fmt.Sprintf("update mysql.tidb set variable_value=%d where variable_name='tidb_server_version'", ver300))
+	mustExec(t, seV3, fmt.Sprintf("delete from mysql.GLOBAL_VARIABLES where variable_name='%s'", variable.TiDBEnableIndexMerge))
+	mustExec(t, seV3, "commit")
+	unsetStoreBootstrapped(store.UUID())
+	ver, err := getBootstrapVersion(seV3)
+	require.NoError(t, err)
+	require.Equal(t, int64(ver300), ver)
+
+	// We are now in 3.0.0, check tidb_enable_index_merge shoudle not exist.
+	res := mustExec(t, seV3, fmt.Sprintf("select * from mysql.GLOBAL_VARIABLES where variable_name='%s'", variable.TiDBEnableIndexMerge))
+	chk := res.NewChunk(nil)
+	err = res.Next(ctx, chk)
+	require.NoError(t, err)
+	require.Equal(t, 0, chk.NumRows())
+
+	domCurVer, err := BootstrapSession(store)
+	require.NoError(t, err)
+	defer domCurVer.Close()
+	seCurVer := createSessionAndSetID(t, store)
+	ver, err = getBootstrapVersion(seCurVer)
+	require.NoError(t, err)
+	require.Equal(t, currentBootstrapVersion, ver)
+
+	// We are now in 5.x, tidb_enable_index_merge should be off.
+	res = mustExec(t, seCurVer, "select @@tidb_enable_index_merge")
+	chk = res.NewChunk(nil)
+	err = res.Next(ctx, chk)
+	require.NoError(t, err)
+	require.Equal(t, 1, chk.NumRows())
+	row := chk.GetRow(0)
+	require.Equal(t, 1, row.Len())
+	require.Equal(t, int64(0), row.GetInt64(0))
+}
+
+func TestIndexMergeUpgradeFrom400To540(t *testing.T) {
+	for i := 0; i < 2; i++ {
+		ctx := context.Background()
+		store, _ := createStoreAndBootstrap(t)
+		defer func() { require.NoError(t, store.Close()) }()
+
+		// upgrade from 4.0.0 to 5.4+.
+		ver400 := 46
+		seV4 := createSessionAndSetID(t, store)
+		txn, err := store.Begin()
+		require.NoError(t, err)
+		m := meta.NewMeta(txn)
+		err = m.FinishBootstrap(int64(ver400))
+		require.NoError(t, err)
+		err = txn.Commit(context.Background())
+		require.NoError(t, err)
+		mustExec(t, seV4, fmt.Sprintf("update mysql.tidb set variable_value=%d where variable_name='tidb_server_version'", ver400))
+		mustExec(t, seV4, fmt.Sprintf("update mysql.GLOBAL_VARIABLES set variable_value='%s' where variable_name='%s'", variable.Off, variable.TiDBEnableIndexMerge))
+		mustExec(t, seV4, "commit")
+		unsetStoreBootstrapped(store.UUID())
+		ver, err := getBootstrapVersion(seV4)
+		require.NoError(t, err)
+		require.Equal(t, int64(ver400), ver)
+
+		// We are now in 4.0.0, tidb_enable_index_merge is off.
+		res := mustExec(t, seV4, fmt.Sprintf("select * from mysql.GLOBAL_VARIABLES where variable_name='%s'", variable.TiDBEnableIndexMerge))
+		chk := res.NewChunk(nil)
+		err = res.Next(ctx, chk)
+		require.NoError(t, err)
+		require.Equal(t, 1, chk.NumRows())
+		row := chk.GetRow(0)
+		require.Equal(t, 2, row.Len())
+		require.Equal(t, variable.Off, row.GetString(1))
+
+		if i == 0 {
+			// For the first time, We set tidb_enable_index_merge as on.
+			// And after upgrade to 5.x, tidb_enable_index_merge should remains to be on.
+			// For the second it should be off.
+			mustExec(t, seV4, "set global tidb_enable_index_merge = on")
+		}
+
+		// Upgrade to 5.x.
+		domCurVer, err := BootstrapSession(store)
+		require.NoError(t, err)
+		defer domCurVer.Close()
+		seCurVer := createSessionAndSetID(t, store)
+		ver, err = getBootstrapVersion(seCurVer)
+		require.NoError(t, err)
+		require.Equal(t, currentBootstrapVersion, ver)
+
+		// We are now in 5.x, tidb_enable_index_merge should be on because we enable it in 4.0.0.
+		res = mustExec(t, seCurVer, "select @@tidb_enable_index_merge")
+		chk = res.NewChunk(nil)
+		err = res.Next(ctx, chk)
+		require.NoError(t, err)
+		require.Equal(t, 1, chk.NumRows())
+		row = chk.GetRow(0)
+		require.Equal(t, 1, row.Len())
+		if i == 0 {
+			require.Equal(t, int64(1), row.GetInt64(0))
+		} else {
+			require.Equal(t, int64(0), row.GetInt64(0))
+		}
+	}
+}

sessionctx/variable/session.go

@@ -1172,7 +1172,7 @@ func NewSessionVars() *SessionVars {
 		SlowQueryFile:               config.GetGlobalConfig().Log.SlowQueryFile,
 		WaitSplitRegionFinish:       DefTiDBWaitSplitRegionFinish,
 		WaitSplitRegionTimeout:      DefWaitSplitRegionTimeout,
-		enableIndexMerge:            false,
+		enableIndexMerge:            DefTiDBEnableIndexMerge,
 		NoopFuncsMode:               TiDBOptOnOffWarn(DefTiDBEnableNoopFuncs),
 		replicaRead:                 kv.ReplicaReadLeader,
 		AllowRemoveAutoInc:          DefTiDBAllowRemoveAutoInc,

sessionctx/variable/sysvar.go

@@ -537,7 +537,7 @@ var defaultSysVars = []*SysVar{
 		s.SetEnableCascadesPlanner(TiDBOptOn(val))
 		return nil
 	}},
-	{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnableIndexMerge, Value: Off, Type: TypeBool, SetSession: func(s *SessionVars, val string) error {
+	{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnableIndexMerge, Value: BoolToOnOff(DefTiDBEnableIndexMerge), Type: TypeBool, SetSession: func(s *SessionVars, val string) error {
 		s.SetEnableIndexMerge(TiDBOptOn(val))
 		return nil
 	}},

sessionctx/variable/sysvar_test.go

@@ -825,3 +825,12 @@ func TestDefaultCharsetAndCollation(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, val, mysql.DefaultCollationName)
 }
+
+func TestIndexMergeSwitcher(t *testing.T) {
+	vars := NewSessionVars()
+	vars.GlobalVarsAccessor = NewMockGlobalAccessor4Tests()
+	val, err := GetSessionOrGlobalSystemVar(vars, TiDBEnableIndexMerge)
+	require.NoError(t, err)
+	require.Equal(t, DefTiDBEnableIndexMerge, true)
+	require.Equal(t, BoolToOnOff(DefTiDBEnableIndexMerge), val)
+}

sessionctx/variable/tidb_vars.go

@@ -779,6 +779,7 @@ const (
 	DefTiDBRegardNULLAsPoint              = true
 	DefEnablePlacementCheck               = true
 	DefTimestamp                          = "0"
+	DefTiDBEnableIndexMerge               = true
 )
 
 // Process global variables.