[memo] variable to disable merge join

enginetest/join_planning_tests.go

@@ -43,11 +43,13 @@ type JoinPlanTest struct {
 	skipOld bool
 }
 
-var JoinPlanningTests = []struct {
+type joinPlanScript struct {
 	name  string
 	setup []string
 	tests []JoinPlanTest
-}{
+}
+
+var JoinPlanningTests = []joinPlanScript{
 	{
 		name: "filter pushdown through join uppercase name",
 		setup: []string{
@@ -78,6 +80,33 @@ var JoinPlanningTests = []struct {
 			},
 		},
 	},
+	{
+		name: "block merge join",
+		setup: []string{
+			"CREATE table xy (x int primary key, y int, unique index y_idx(y));",
+			"CREATE table ab (a int primary key, b int);",
+			"insert into xy values (1,0), (2,1), (0,2), (3,3);",
+			"insert into ab values (0,2), (1,2), (2,2), (3,1);",
+			`analyze table xy update histogram on x using data '{"row_count":1000}'`,
+			`analyze table ab update histogram on a using data '{"row_count":1000}'`,
+		},
+		tests: []JoinPlanTest{
+			{
+				q:     "select /*+ JOIN_ORDER(ab, xy) MERGE_JOIN(ab, xy)*/ * from ab join xy on y = a order by 1, 3",
+				types: []plan.JoinType{plan.JoinTypeMerge},
+				exp:   []sql.Row{{0, 2, 1, 0}, {1, 2, 2, 1}, {2, 2, 0, 2}, {3, 1, 3, 3}},
+			},
+			{
+				q:   "set @@SESSION.disable_merge_join = 1",
+				exp: []sql.Row{{}},
+			},
+			{
+				q:     "select /*+ JOIN_ORDER(ab, xy) MERGE_JOIN(ab, xy)*/ * from ab join xy on y = a order by 1, 3",
+				types: []plan.JoinType{plan.JoinTypeLookup},
+				exp:   []sql.Row{{0, 2, 1, 0}, {1, 2, 2, 1}, {2, 2, 0, 2}, {3, 1, 3, 3}},
+			},
+		},
+	},
 	{
 		name: "merge join unary index",
 		setup: []string{
@@ -1725,7 +1754,11 @@ join uv d on d.u = c.x`,
 }
 
 func TestJoinPlanning(t *testing.T, harness Harness) {
-	for _, tt := range JoinPlanningTests {
+	runJoinPlanningTests(t, harness, JoinPlanningTests)
+}
+
+func runJoinPlanningTests(t *testing.T, harness Harness, tests []joinPlanScript) {
+	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			harness.Setup([]setup.SetupScript{setup.MydbData[0], tt.setup})
 			e := mustNewEngine(t, harness)
@@ -1750,7 +1783,6 @@ func TestJoinPlanning(t *testing.T, harness Harness) {
 		})
 	}
 }
-
 func evalJoinTypeTest(t *testing.T, harness Harness, e QueryEngine, query string, types []plan.JoinType, skipOld bool) {
 	t.Run(query+" join types", func(t *testing.T) {
 		if skipOld {

sql/analyzer/indexed_joins.go

@@ -200,6 +200,7 @@ func replanJoin(ctx *sql.Context, n *plan.JoinNode, a *Analyzer, scope *plan.Sco
 		return nil, err
 	}
 
+	m.SetDefaultHints()
 	hints := memo.ExtractJoinHint(n)
 	for _, h := range hints {
 		// this should probably happen earlier, but the root is not

sql/memo/memo.go

@@ -82,6 +82,12 @@ func (m *Memo) StatsProvider() sql.StatsProvider {
 	return m.statsProv
 }
 
+func (m *Memo) SetDefaultHints() {
+	if val, _ := m.Ctx.GetSessionVariable(m.Ctx, sql.DisableMergeJoin); val.(int8) != 0 {
+		m.ApplyHint(Hint{Typ: HintTypeNoMergeJoin})
+	}
+}
+
 // newExprGroup creates a new logical expression group to encapsulate the
 // action of a SQL clause.
 // TODO: this is supposed to deduplicate logically equivalent table scans
@@ -459,6 +465,11 @@ func (m *Memo) optimizeMemoGroup(grp *ExprGroup) error {
 // rather than a local property.
 func (m *Memo) updateBest(grp *ExprGroup, n RelExpr, cost float64) {
 	if !m.hints.isEmpty() {
+		for _, block := range m.hints.block {
+			if !block.isOk(n) {
+				return
+			}
+		}
 		if m.hints.satisfiedBy(n) {
 			if !grp.HintOk {
 				grp.Best = n
@@ -514,17 +525,32 @@ func getProjectColset(p *Project) sql.ColSet {
 func (m *Memo) ApplyHint(hint Hint) {
 	switch hint.Typ {
 	case HintTypeJoinOrder:
-		m.WithJoinOrder(hint.Args)
+		m.SetJoinOrder(hint.Args)
 	case HintTypeJoinFixedOrder:
+	case HintTypeNoMergeJoin:
+		m.SetBlockOp(func(n RelExpr) bool {
+			switch n := n.(type) {
+			case JoinRel:
+				jp := n.JoinPrivate()
+				if !jp.Left.Best.Group().HintOk || !jp.Right.Best.Group().HintOk {
+					// equiv closures can generate child plans that bypass hints
+					return false
+				}
+				if jp.Op.IsMerge() {
+					return false
+				}
+			}
+			return true
+		})
 	case HintTypeInnerJoin, HintTypeMergeJoin, HintTypeLookupJoin, HintTypeHashJoin, HintTypeSemiJoin, HintTypeAntiJoin, HintTypeLeftOuterLookupJoin:
-		m.WithJoinOp(hint.Typ, hint.Args[0], hint.Args[1])
+		m.SetJoinOp(hint.Typ, hint.Args[0], hint.Args[1])
 	case HintTypeLeftDeep:
 		m.hints.leftDeep = true
 	default:
 	}
 }
 
-func (m *Memo) WithJoinOrder(tables []string) {
+func (m *Memo) SetJoinOrder(tables []string) {
 	// order maps groupId -> table dependencies
 	order := make(map[sql.TableId]uint64)
 	for i, t := range tables {
@@ -542,7 +568,11 @@ func (m *Memo) WithJoinOrder(tables []string) {
 	}
 }
 
-func (m *Memo) WithJoinOp(op HintType, left, right string) {
+func (m *Memo) SetBlockOp(cb func(n RelExpr) bool) {
+	m.hints.block = append(m.hints.block, joinBlockHint{cb: cb})
+}
+
+func (m *Memo) SetJoinOp(op HintType, left, right string) {
 	var lTab, rTab sql.TableId
 	for _, n := range m.root.RelProps.TableIdNodes() {
 		if strings.EqualFold(left, n.Name()) {

sql/memo/select_hints.go

@@ -32,6 +32,7 @@ const (
 	HintTypeUnknown                  HintType = iota //
 	HintTypeJoinOrder                                // JOIN_ORDER
 	HintTypeJoinFixedOrder                           // JOIN_FIXED_ORDER
+	HintTypeNoMergeJoin                              // NO_MERGE_JOIN
 	HintTypeMergeJoin                                // MERGE_JOIN
 	HintTypeLookupJoin                               // LOOKUP_JOIN
 	HintTypeHashJoin                                 // HASH_JOIN
@@ -81,6 +82,8 @@ func newHint(joinTyp string, args []string) Hint {
 		typ = HintTypeNoIndexConditionPushDown
 	case "left_deep":
 		typ = HintTypeLeftDeep
+	case "no_merge_join":
+		typ = HintTypeNoMergeJoin
 	default:
 		typ = HintTypeUnknown
 	}
@@ -111,6 +114,8 @@ func (h Hint) valid() bool {
 		return len(h.Args) == 0
 	case HintTypeLeftDeep:
 		return len(h.Args) == 0
+	case HintTypeNoMergeJoin:
+		return true
 	case HintTypeUnknown:
 		return false
 	default:
@@ -367,11 +372,20 @@ func (o joinOpHint) typeMatches(n RelExpr) bool {
 	return true
 }
 
+type joinBlockHint struct {
+	cb func(n RelExpr) bool
+}
+
+func (o joinBlockHint) isOk(n RelExpr) bool {
+	return o.cb(n)
+}
+
 // joinHints wraps a collection of join hints. The memo
 // interfaces with this object during costing.
 type joinHints struct {
 	ops      []joinOpHint
 	order    *joinOrderHint
+	block    []joinBlockHint
 	leftDeep bool
 }
 

sql/memo/select_hints_test.go

@@ -238,7 +238,7 @@ func TestOrderHintBuilding(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			j := NewJoinOrderBuilder(NewMemo(newContext(pro), nil, nil, 0, NewDefaultCoster(), nil))
 			j.ReorderJoin(tt.plan)
-			j.m.WithJoinOrder(tt.hint)
+			j.m.SetJoinOrder(tt.hint)
 			if tt.invalid {
 				require.Equal(t, j.m.hints.order, (*joinOrderHint)(nil))
 			} else {

sql/statistics.go

@@ -21,6 +21,8 @@ import (
 	"time"
 )
 
+const DisableMergeJoin = "disable_merge_join"
+
 // StatisticsTable is a table that can provide information about its number of rows and other facts to improve query
 // planning performance.
 type StatisticsTable interface {

sql/variables/system_variables.go

@@ -1049,6 +1049,14 @@ var systemVars = map[string]sql.SystemVariable{
 		Type:              types.NewSystemBoolType("inmemory_joins"),
 		Default:           int8(0),
 	},
+	"disable_merge_join": &sql.MysqlSystemVariable{
+		Name:              sql.DisableMergeJoin,
+		Scope:             sql.GetMysqlScope(sql.SystemVariableScope_Both),
+		Dynamic:           true,
+		SetVarHintApplies: false,
+		Type:              types.NewSystemBoolType(sql.DisableMergeJoin),
+		Default:           int8(0),
+	},
 	"innodb_autoinc_lock_mode": &sql.MysqlSystemVariable{
 		Name:              "innodb_autoinc_lock_mode",
 		Scope:             sql.GetMysqlScope(sql.SystemVariableScope_Global),