diff --git a/executor/builder.go b/executor/builder.go
index 993521d58fd6b..3490188646b74 100644
--- a/executor/builder.go
+++ b/executor/builder.go
@@ -2184,10 +2184,11 @@ func (builder *dataReaderBuilder) buildIndexLookUpReaderForIndexJoin(ctx context
 
 // buildKvRangesForIndexJoin builds kv ranges for index join when the inner plan is index scan plan.
 func buildKvRangesForIndexJoin(ctx sessionctx.Context, tableID, indexID int64, lookUpContents []*indexJoinLookUpContent,
-	ranges []*ranger.Range, keyOff2IdxOff []int, cwc *plannercore.ColWithCmpFuncManager) ([]kv.KeyRange, error) {
+	ranges []*ranger.Range, keyOff2IdxOff []int, cwc *plannercore.ColWithCmpFuncManager) (_ []kv.KeyRange, err error) {
 	kvRanges := make([]kv.KeyRange, 0, len(ranges)*len(lookUpContents))
 	lastPos := len(ranges[0].LowVal) - 1
 	sc := ctx.GetSessionVars().StmtCtx
+	tmpDatumRanges := make([]*ranger.Range, 0, len(lookUpContents))
 	for _, content := range lookUpContents {
 		for _, ran := range ranges {
 			for keyOff, idxOff := range keyOff2IdxOff {
@@ -2195,54 +2196,41 @@ func buildKvRangesForIndexJoin(ctx sessionctx.Context, tableID, indexID int64, l
 				ran.HighVal[idxOff] = content.keys[keyOff]
 			}
 		}
-		if cwc != nil {
-			nextColRanges, err := cwc.BuildRangesByRow(ctx, content.row)
+		if cwc == nil {
+			tmpKvRanges, err := distsql.IndexRangesToKVRanges(sc, tableID, indexID, ranges, nil)
 			if err != nil {
 				return nil, err
 			}
-			for _, nextColRan := range nextColRanges {
-				for _, ran := range ranges {
-					ran.LowVal[lastPos] = nextColRan.LowVal[0]
-					ran.HighVal[lastPos] = nextColRan.HighVal[0]
-					ran.LowExclude = nextColRan.LowExclude
-					ran.HighExclude = nextColRan.HighExclude
-				}
-				tmpKvRanges, err := distsql.IndexRangesToKVRanges(sc, tableID, indexID, ranges, nil)
-				if err != nil {
-					return nil, errors.Trace(err)
-				}
-				kvRanges = append(kvRanges, tmpKvRanges...)
-			}
+			kvRanges = append(kvRanges, tmpKvRanges...)
 			continue
 		}
-
-		tmpKvRanges, err := distsql.IndexRangesToKVRanges(sc, tableID, indexID, ranges, nil)
+		nextColRanges, err := cwc.BuildRangesByRow(ctx, content.row)
 		if err != nil {
 			return nil, err
 		}
-		kvRanges = append(kvRanges, tmpKvRanges...)
-	}
-	// Sort and merge the overlapped ranges.
-	sort.Slice(kvRanges, func(i, j int) bool {
-		return bytes.Compare(kvRanges[i].StartKey, kvRanges[j].StartKey) < 0
-	})
-	if cwc != nil {
-		// If cwc is not nil, we need to merge the overlapped ranges here.
-		mergedKeyRanges := make([]kv.KeyRange, 0, len(kvRanges))
-		for i := range kvRanges {
-			if len(mergedKeyRanges) == 0 {
-				mergedKeyRanges = append(mergedKeyRanges, kvRanges[i])
-				continue
-			}
-			if bytes.Compare(kvRanges[i].StartKey, mergedKeyRanges[len(mergedKeyRanges)-1].EndKey) <= 0 {
-				mergedKeyRanges[len(mergedKeyRanges)-1].EndKey = kvRanges[i].EndKey
-			} else {
-				mergedKeyRanges = append(mergedKeyRanges, kvRanges[i])
+		for _, nextColRan := range nextColRanges {
+			for _, ran := range ranges {
+				ran.LowVal[lastPos] = nextColRan.LowVal[0]
+				ran.HighVal[lastPos] = nextColRan.HighVal[0]
+				ran.LowExclude = nextColRan.LowExclude
+				ran.HighExclude = nextColRan.HighExclude
+				tmpDatumRanges = append(tmpDatumRanges, ran.Clone())
 			}
 		}
-		return mergedKeyRanges, nil
 	}
-	return kvRanges, nil
+
+	if cwc == nil {
+		sort.Slice(kvRanges, func(i, j int) bool {
+			return bytes.Compare(kvRanges[i].StartKey, kvRanges[j].StartKey) < 0
+		})
+		return kvRanges, nil
+	}
+
+	tmpDatumRanges, err = ranger.UnionRanges(ctx.GetSessionVars().StmtCtx, tmpDatumRanges)
+	if err != nil {
+		return nil, err
+	}
+	return distsql.IndexRangesToKVRanges(ctx.GetSessionVars().StmtCtx, tableID, indexID, tmpDatumRanges, nil)
 }
 
 func (b *executorBuilder) buildWindow(v *plannercore.PhysicalWindow) *WindowExec {
diff --git a/executor/join_test.go b/executor/join_test.go
index 9d80b6b744bc3..0d5bb3e1fa838 100644
--- a/executor/join_test.go
+++ b/executor/join_test.go
@@ -916,6 +916,19 @@ func (s *testSuite2) TestIndexLookupJoin(c *C) {
 	tk.MustQuery("select /*+ inl_merge_join(t1)*/ * from t1 join t2 on t2.b=t1.id and t2.a=t1.id;").Check(testkit.Rows("1 1 1"))
 }
 
+func (s *testSuite2) TestIssue15686(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t, k;")
+	tk.MustExec("create table k (a int, pk int primary key, index(a));")
+	tk.MustExec("create table t (a int, pk int primary key, index(a));")
+	tk.MustExec("insert into k values(0,8),(0,23),(1,21),(1,33),(1,52),(2,17),(2,34),(2,39),(2,40),(2,66),(2,67),(3,9),(3,25),(3,41),(3,48),(4,4),(4,11),(4,15),(4,26),(4,27),(4,31),(4,35),(4,45),(4,47),(4,49);")
+	tk.MustExec("insert into t values(3,4),(3,5),(3,27),(3,29),(3,57),(3,58),(3,79),(3,84),(3,92),(3,95);")
+	tk.MustQuery("select /*+ inl_join(t) */ count(*) from k left join t on k.a = t.a and k.pk > t.pk;").Check(testkit.Rows("33"))
+	tk.MustQuery("select /*+ inl_hash_join(t) */ count(*) from k left join t on k.a = t.a and k.pk > t.pk;").Check(testkit.Rows("33"))
+	tk.MustQuery("select /*+ inl_merge_join(t) */ count(*) from k left join t on k.a = t.a and k.pk > t.pk;").Check(testkit.Rows("33"))
+}
+
 func (s *testSuite2) TestMergejoinOrder(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")
diff --git a/util/ranger/detacher.go b/util/ranger/detacher.go
index 671ea328a0c5b..42e6d720549fd 100644
--- a/util/ranger/detacher.go
+++ b/util/ranger/detacher.go
@@ -306,7 +306,7 @@ func detachDNFCondAndBuildRangeForIndex(sctx sessionctx.Context, condition *expr
 		}
 	}
 
-	totalRanges, err := unionRanges(sc, totalRanges)
+	totalRanges, err := UnionRanges(sc, totalRanges)
 	if err != nil {
 		return nil, nil, false, errors.Trace(err)
 	}
diff --git a/util/ranger/ranger.go b/util/ranger/ranger.go
index 6f858af480a39..ae754582e9ef6 100644
--- a/util/ranger/ranger.go
+++ b/util/ranger/ranger.go
@@ -270,7 +270,7 @@ func buildColumnRange(accessConditions []expression.Expression, sc *stmtctx.Stat
 				ran.HighExclude = false
 			}
 		}
-		ranges, err = unionRanges(sc, ranges)
+		ranges, err = UnionRanges(sc, ranges)
 		if err != nil {
 			return nil, err
 		}
@@ -340,7 +340,7 @@ func buildCNFIndexRange(sc *stmtctx.StatementContext, cols []*expression.Column,
 	// Take prefix index into consideration.
 	if hasPrefix(lengths) {
 		if fixPrefixColRange(ranges, lengths, newTp) {
-			ranges, err = unionRanges(sc, ranges)
+			ranges, err = UnionRanges(sc, ranges)
 			if err != nil {
 				return nil, errors.Trace(err)
 			}
@@ -356,7 +356,11 @@ type sortRange struct {
 	encodedEnd    []byte
 }
 
-func unionRanges(sc *stmtctx.StatementContext, ranges []*Range) ([]*Range, error) {
+// UnionRanges sorts `ranges`, union adjacent ones if possible.
+// For two intervals [a, b], [c, d], we have guaranteed that a <= c. If b >= c. Then two intervals are overlapped.
+// And this two can be merged as [a, max(b, d)].
+// Otherwise they aren't overlapped.
+func UnionRanges(sc *stmtctx.StatementContext, ranges []*Range) ([]*Range, error) {
 	if len(ranges) == 0 {
 		return nil, nil
 	}
@@ -384,9 +388,6 @@ func unionRanges(sc *stmtctx.StatementContext, ranges []*Range) ([]*Range, error
 	ranges = ranges[:0]
 	lastRange := objects[0]
 	for i := 1; i < len(objects); i++ {
-		// For two intervals [a, b], [c, d], we have guaranteed that a >= c. If b >= c. Then two intervals are overlapped.
-		// And this two can be merged as [a, max(b, d)].
-		// Otherwise they aren't overlapped.
 		if bytes.Compare(lastRange.encodedEnd, objects[i].encodedStart) >= 0 {
 			if bytes.Compare(lastRange.encodedEnd, objects[i].encodedEnd) < 0 {
 				lastRange.encodedEnd = objects[i].encodedEnd