planner: fix panic of merge join for tables with redundant indexes (p…

…ingcap#15840)

planner/core/exhaust_physical_plans.go

@@ -85,7 +85,7 @@ func (p *LogicalJoin) moveEqualToOtherConditions(offsets []int) []expression.Exp
 
 	// Construct otherConds, which is composed of the original other conditions
 	// and the remained unused equal conditions.
-	numOtherConds := len(p.OtherConditions) + len(p.EqualConditions) - len(offsets)
+	numOtherConds := len(p.OtherConditions) + len(p.EqualConditions) - len(usedEqConds)
 	otherConds := make([]expression.Expression, len(p.OtherConditions), numOtherConds)
 	copy(otherConds, p.OtherConditions)
 	for eqCondIdx := range p.EqualConditions {

planner/core/integration_test.go

@@ -711,6 +711,18 @@ func (s *testIntegrationSuite) TestIssue15546(c *C) {
 	tk.MustQuery("select * from pt, vt where pt.a = vt.a").Check(testkit.Rows("1 1 1 1"))
 }
 
+func (s *testIntegrationSuite) TestIssue15813(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t0, t1")
+	tk.MustExec("create table t0(c0 int primary key)")
+	tk.MustExec("create table t1(c0 int primary key)")
+	tk.MustExec("CREATE INDEX i0 ON t0(c0)")
+	tk.MustExec("CREATE INDEX i0 ON t1(c0)")
+	tk.MustQuery("select /*+ MERGE_JOIN(t0, t1) */ * from t0, t1 where t0.c0 = t1.c0").Check(testkit.Rows())
+}
+
 func (s *testIntegrationSuite) TestHintWithoutTableWarning(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")