diff --git a/executor/merge_join.go b/executor/merge_join.go
index 7939d128d9948..46bf9bf0b5e53 100644
--- a/executor/merge_join.go
+++ b/executor/merge_join.go
@@ -88,7 +88,7 @@ type mergeJoinInnerTable struct {
 }
 
 func (t *mergeJoinInnerTable) init(ctx context.Context, chk4Reader *chunk.Chunk) (err error) {
-	if t.reader == nil || t.joinKeys == nil || len(t.joinKeys) == 0 || ctx == nil {
+	if t.reader == nil || ctx == nil {
 		return errors.Errorf("Invalid arguments: Empty arguments detected.")
 	}
 	t.ctx = ctx
diff --git a/executor/merge_join_test.go b/executor/merge_join_test.go
index 899164094630c..fce712be0dbc1 100644
--- a/executor/merge_join_test.go
+++ b/executor/merge_join_test.go
@@ -236,7 +236,6 @@ func checkPlanAndRun(tk *testkit.TestKit, c *C, plan string, sql string) *testki
 }
 
 func (s *testSuite) TestMergeJoin(c *C) {
-	// FIXME: the TIDB_SMJ hint does not really work when there is no index on join onCondition.
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")
 
@@ -331,6 +330,25 @@ func (s *testSuite) TestMergeJoin(c *C) {
 	tk.MustExec("create table s(a int, primary key(a))")
 	tk.MustExec("insert into s value(1)")
 	tk.MustQuery("select /*+ TIDB_SMJ(t, s) */ count(*) from t join s on t.a = s.a").Check(testkit.Rows("4"))
+
+	// Test TIDB_SMJ for cartesian product.
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int)")
+	tk.MustExec("insert into t value(1),(2)")
+	tk.MustQuery("explain select /*+ TIDB_SMJ(t1, t2) */ * from t t1 join t t2 order by t1.a, t2.a").Check(testkit.Rows(
+		"Sort_6 100000000.00 root t1.a:asc, t2.a:asc",
+		"└─MergeJoin_9 100000000.00 root inner join",
+		"  ├─TableReader_11 10000.00 root data:TableScan_10",
+		"  │ └─TableScan_10 10000.00 cop table:t1, range:[-inf,+inf], keep order:false, stats:pseudo",
+		"  └─TableReader_13 10000.00 root data:TableScan_12",
+		"    └─TableScan_12 10000.00 cop table:t2, range:[-inf,+inf], keep order:false, stats:pseudo",
+	))
+	tk.MustQuery("select /*+ TIDB_SMJ(t1, t2) */ * from t t1 join t t2 order by t1.a, t2.a").Check(testkit.Rows(
+		"1 1",
+		"1 2",
+		"2 1",
+		"2 2",
+	))
 }
 
 func (s *testSuite) Test3WaysMergeJoin(c *C) {
diff --git a/planner/core/exhaust_physical_plans.go b/planner/core/exhaust_physical_plans.go
index dd076d3f9ce52..5f9b181369772 100644
--- a/planner/core/exhaust_physical_plans.go
+++ b/planner/core/exhaust_physical_plans.go
@@ -588,12 +588,16 @@ func (p *LogicalJoin) buildFakeEqCondsForIndexJoin(keys, idxCols []*expression.C
 // 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) ([]PhysicalPlan, bool) {
-	if len(p.EqualConditions) == 0 {
-		return nil, false
-	}
 	plans := make([]PhysicalPlan, 0, 2)
 	rightOuter := (p.preferJoinType & preferLeftAsIndexInner) > 0
 	leftOuter := (p.preferJoinType & preferRightAsIndexInner) > 0
+	if len(p.EqualConditions) == 0 {
+		if leftOuter || rightOuter {
+			warning := ErrInternal.GenWithStack("TIDB_INLJ hint is inapplicable without column equal ON condition")
+			p.ctx.GetSessionVars().StmtCtx.AppendWarning(warning)
+		}
+		return nil, false
+	}
 	switch p.JoinType {
 	case SemiJoin, AntiSemiJoin, LeftOuterSemiJoin, AntiLeftOuterSemiJoin, LeftOuterJoin:
 		join := p.getIndexJoinByOuterIdx(prop, 0)
diff --git a/planner/core/physical_plan_test.go b/planner/core/physical_plan_test.go
index eb21d01215066..b5d0246388ac6 100644
--- a/planner/core/physical_plan_test.go
+++ b/planner/core/physical_plan_test.go
@@ -22,6 +22,7 @@ import (
 	"github.com/pingcap/tidb/planner/core"
 	"github.com/pingcap/tidb/session"
 	"github.com/pingcap/tidb/sessionctx"
+	"github.com/pingcap/tidb/terror"
 	"github.com/pingcap/tidb/util/testleak"
 	"golang.org/x/net/context"
 )
@@ -1280,3 +1281,27 @@ func (s *testPlanSuite) TestRequestTypeSupportedOff(c *C) {
 	c.Assert(err, IsNil)
 	c.Assert(core.ToString(p), Equals, expect, Commentf("for %s", sql))
 }
+
+func (s *testPlanSuite) TestIndexLookupCartesianJoin(c *C) {
+	defer testleak.AfterTest(c)()
+	store, dom, err := newStoreWithBootstrap()
+	c.Assert(err, IsNil)
+	defer func() {
+		dom.Close()
+		store.Close()
+	}()
+	se, err := session.CreateSession4Test(store)
+	c.Assert(err, IsNil)
+	_, err = se.Execute(context.Background(), "use test")
+	c.Assert(err, IsNil)
+	sql := "select /*+ TIDB_INLJ(t1, t2) */ * from t t1 join t t2"
+	stmt, err := s.ParseOneStmt(sql, "", "")
+	c.Assert(err, IsNil)
+	p, err := core.Optimize(se, stmt, s.is)
+	c.Assert(err, IsNil)
+	c.Assert(core.ToString(p), Equals, "LeftHashJoin{TableReader(Table(t))->TableReader(Table(t))}")
+	warnings := se.GetSessionVars().StmtCtx.GetWarnings()
+	lastWarn := warnings[len(warnings)-1]
+	err = core.ErrInternal.GenWithStack("TIDB_INLJ hint is inapplicable without column equal ON condition")
+	c.Assert(terror.ErrorEqual(err, lastWarn.Err), IsTrue)
+}