statistics: improve estimation for index equal condition (#17366) (#1…

…7609)

Signed-off-by: sre-bot <sre-bot@pingcap.com>

planner/core/cbo_test.go

@@ -1050,3 +1050,32 @@ func (s *testAnalyzeSuite) TestUpdateProjEliminate(c *C) {
 	tk.MustExec("create table t(a int, b int)")
 	tk.MustExec("explain update t t1, (select distinct b from t) t2 set t1.b = t2.b")
 }
+
+func (s *testAnalyzeSuite) TestIndexEqualUnknown(c *C) {
+	defer testleak.AfterTest(c)()
+	store, dom, err := newStoreWithBootstrap()
+	c.Assert(err, IsNil)
+	testKit := testkit.NewTestKit(c, store)
+	defer func() {
+		dom.Close()
+		store.Close()
+	}()
+	testKit.MustExec("use test")
+	testKit.MustExec("drop table if exists t, t1")
+	testKit.MustExec("CREATE TABLE t(a bigint(20) NOT NULL, b bigint(20) NOT NULL, c bigint(20) NOT NULL, PRIMARY KEY (a,c,b), KEY (b))")
+	err = s.loadTableStats("analyzeSuiteTestIndexEqualUnknownT.json", dom)
+	c.Assert(err, IsNil)
+	var input []string
+	var output []struct {
+		SQL  string
+		Plan []string
+	}
+	s.testData.GetTestCases(c, &input, &output)
+	for i, tt := range input {
+		s.testData.OnRecord(func() {
+			output[i].SQL = tt
+			output[i].Plan = s.testData.ConvertRowsToStrings(testKit.MustQuery(tt).Rows())
+		})
+		testKit.MustQuery(tt).Check(testkit.Rows(output[i].Plan...))
+	}
+}