planner: fix bug some cases index merge join hint doesn't work

planner/core/exhaust_physical_plans.go

@@ -1264,7 +1264,7 @@ func (ijHelper *indexJoinBuildHelper) buildTemplateRange(matchedKeyCnt int, eqAn
 
 // tryToGetIndexJoin will get index join by hints. If we can generate a valid index join by hint, the second return value
 // will be true, which means we force to choose this index join. Otherwise we will select a join algorithm with min-cost.
-func (p *LogicalJoin) tryToGetIndexJoin(prop *property.PhysicalProperty) (indexJoins []PhysicalPlan, forced bool) {
+func (p *LogicalJoin) tryToGetIndexJoin(prop *property.PhysicalProperty) (indexJoins []PhysicalPlan, canForced bool) {
 	inljRightOuter := (p.preferJoinType & preferLeftAsINLJInner) > 0
 	inljLeftOuter := (p.preferJoinType & preferRightAsINLJInner) > 0
 	hasINLJHint := inljLeftOuter || inljRightOuter
@@ -1279,10 +1279,29 @@ func (p *LogicalJoin) tryToGetIndexJoin(prop *property.PhysicalProperty) (indexJ
 
 	forceLeftOuter := inljLeftOuter || inlhjLeftOuter || inlmjLeftOuter
 	forceRightOuter := inljRightOuter || inlhjRightOuter || inlmjRightOuter
+	needForced := forceLeftOuter || forceRightOuter
 
 	defer func() {
 		// refine error message
-		if !forced && (hasINLJHint || hasINLHJHint || hasINLMJHint) {
+		if !canForced && needForced {
+			if hasINLMJHint && len(indexJoins) > 0 && len(prop.Items) > 0 {
+				containIdxMergeJoin := false
+				for _, idxJoin := range indexJoins {
+					if _, ok := idxJoin.(*PhysicalIndexMergeJoin); ok {
+						containIdxMergeJoin = true
+						break
+					}
+				}
+				// 1. IndexMergeJoin requires stricter conditions than Index(Hash)Join when the output order is needed.
+				// 2. IndexMergeJoin requires the same conditions with Index(Hash)Join when the output is unordered.
+				// 3. If ordered-Index(Hash)Join can be chosen but ordered-IndexMergeJoin can not be chosen, we can build a plan with an enforced sort on IndexMergeJoin.
+				// 4. Thus we can give up the plans here if IndexMergeJoin is nil when `hasINLMJHint` is true. Because we can make sure that an IndexMeregJoin with enforced sort will be built.
+				if !containIdxMergeJoin {
+					canForced = true
+					indexJoins = nil
+					return
+				}
+			}
 			// Construct warning message prefix.
 			var errMsg string
 			switch {
@@ -1385,8 +1404,8 @@ func (p *LogicalJoin) tryToGetIndexJoin(prop *property.PhysicalProperty) (indexJ
 
 	canForceLeft := len(forcedLeftOuterJoins) != 0 && forceLeftOuter
 	canForceRight := len(forcedRightOuterJoins) != 0 && forceRightOuter
-	forced = canForceLeft || canForceRight
-	if forced {
+	canForced = canForceLeft || canForceRight
+	if canForced {
 		return append(forcedLeftOuterJoins, forcedRightOuterJoins...), true
 	}
 	return append(allLeftOuterJoins, allRightOuterJoins...), false

planner/core/physical_plan_test.go

@@ -1152,12 +1152,14 @@ func (s *testPlanSuite) TestIndexJoinHint(c *C) {
 	ctx := context.Background()
 	_, err = se.Execute(ctx, "use test")
 	c.Assert(err, IsNil)
-	_, err = se.Execute(ctx, `drop table if exists test.t1, test.t2;`)
+	_, err = se.Execute(ctx, `drop table if exists test.t1, test.t2, test.t;`)
 	c.Assert(err, IsNil)
 	_, err = se.Execute(ctx, `create table test.t1(a bigint, b bigint, index idx_a(a), index idx_b(b));`)
 	c.Assert(err, IsNil)
 	_, err = se.Execute(ctx, `create table test.t2(a bigint, b bigint, index idx_a(a), index idx_b(b));`)
 	c.Assert(err, IsNil)
+	_, err = se.Execute(ctx, "CREATE TABLE `t` ( `a` bigint(20) NOT NULL, `b` tinyint(1) DEFAULT NULL, `c` datetime DEFAULT NULL, `d` int(10) unsigned DEFAULT NULL, `e` varchar(20) DEFAULT NULL, `f` double DEFAULT NULL, `g` decimal(30,5) DEFAULT NULL, `h` float DEFAULT NULL, `i` date DEFAULT NULL, `j` timestamp NULL DEFAULT NULL, PRIMARY KEY (`a`), UNIQUE KEY `b` (`b`), KEY `c` (`c`,`d`,`e`), KEY `f` (`f`), KEY `g` (`g`,`h`), KEY `g_2` (`g`), UNIQUE KEY `g_3` (`g`), KEY `i` (`i`) );")
+	c.Assert(err, IsNil)
 	var input []string
 	var output []struct {
 		SQL  string

planner/core/testdata/plan_suite_in.json

@@ -516,7 +516,10 @@
     "cases": [
       "select /*+ INL_JOIN(t1) */ * from t1 join t2 on t1.a = t2.a;",
       "select /*+ INL_HASH_JOIN(t1) */ * from t1 join t2 on t1.a = t2.a;",
-      "select /*+ INL_MERGE_JOIN(t1) */ * from t1 join t2 on t1.a = t2.a;"
+      "select /*+ INL_MERGE_JOIN(t1) */ * from t1 join t2 on t1.a = t2.a;",
+      // Issue 15484
+      "select /*+ inl_merge_join(t2) */ t1.a, t2.a from t t1 left join t t2 use index(g_2) on t1.g=t2.g",
+      "select /*+inl_merge_join(t2)*/ t1.a, t2.a from t t1 left join t t2 use index(g_2) on t1.g=t2.g order by t1.a"
     ]
   },
   {

planner/core/testdata/plan_suite_out.json

@@ -1378,6 +1378,14 @@
       {
         "SQL": "select /*+ INL_MERGE_JOIN(t1) */ * from t1 join t2 on t1.a = t2.a;",
         "Plan": "IndexMergeJoin{IndexLookUp(Index(t1.idx_a)[[NULL,+inf]]->Sel([not(isnull(test.t1.a))]), Table(t1))->Projection->TableReader(Table(t2)->Sel([not(isnull(test.t2.a))]))}(test.t2.a,test.t1.a)"
+      },
+      {
+        "SQL": "select /*+ inl_merge_join(t2) */ t1.a, t2.a from t t1 left join t t2 use index(g_2) on t1.g=t2.g",
+        "Plan": "IndexMergeJoin{IndexReader(Index(t.g_3)[[NULL,+inf]])->IndexReader(Index(t.g_2)[[NULL,+inf]]->Sel([not(isnull(test.t.g))]))}(test.t.g,test.t.g)"
+      },
+      {
+        "SQL": "select /*+inl_merge_join(t2)*/ t1.a, t2.a from t t1 left join t t2 use index(g_2) on t1.g=t2.g order by t1.a",
+        "Plan": "IndexMergeJoin{IndexReader(Index(t.g_3)[[NULL,+inf]])->IndexReader(Index(t.g_2)[[NULL,+inf]]->Sel([not(isnull(test.t.g))]))}(test.t.g,test.t.g)->Sort"
       }
     ]
   },