planner: fix the issue that the optimizer cannot convert OUTER JOIN to INNER JOIN with nested AND/OR in some cases

pkg/planner/core/integration_test.go

@@ -1626,6 +1626,17 @@ func TestLeastGretestStringPushDownToTiFlash(t *testing.T) {
 	tk.MustQuery("explain select greatest(a, b) from t;").CheckAt([]int{0, 2, 4}, rows)
 }
 
+func TestIssue49616(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec(`use test`)
+	tk.MustExec(`create table t1 (k int, a int)`)
+	tk.MustExec(`create table t2 (k int, b int, key(k))`)
+	tk.MustHavePlan(`select /*+ tidb_inlj(t2, t1) */ *
+  from t2 left join t1 on t1.k=t2.k
+  where a>0 or (a=0 and b>0)`, `IndexJoin`)
+}
+
 func TestTiFlashReadForWriteStmt(t *testing.T) {
 	store, dom := testkit.CreateMockStoreAndDomain(t)
 	tk := testkit.NewTestKit(t, store)

pkg/planner/core/rule_predicate_push_down.go

@@ -440,6 +440,10 @@ func isNullRejected(ctx sessionctx.Context, schema *expression.Schema, expr expr
 		sc.InNullRejectCheck = false
 	}()
 	for _, cond := range expression.SplitCNFItems(expr) {
+		if isNullRejectedSpecially(ctx, schema, expr) {
+			return true
+		}
+
 		result := expression.EvaluateExprWithNull(ctx, schema, cond)
 		x, ok := result.(*expression.Constant)
 		if !ok {
@@ -454,6 +458,47 @@ func isNullRejected(ctx sessionctx.Context, schema *expression.Schema, expr expr
 	return false
 }
 
+// isNullRejectedSpecially handles some null-rejected cases specially, since the current in
+// EvaluateExprWithNull is too strict for some cases, e.g. #49616.
+func isNullRejectedSpecially(ctx sessionctx.Context, schema *expression.Schema, expr expression.Expression) bool {
+	return specialNullRejectedCase1(ctx, schema, expr) // only 1 case now
+}
+
+// specialNullRejectedCase1 is mainly for #49616.
+// Case1 specially handles `null-rejected OR (null-rejected AND {others})`, then no matter what the result
+// of `{others}` is (True, False or Null), the result of this predicate is null, so this predicate is null-rejected.
+func specialNullRejectedCase1(ctx sessionctx.Context, schema *expression.Schema, expr expression.Expression) bool {
+	isFunc := func(e expression.Expression, lowerFuncName string) *expression.ScalarFunction {
+		f, ok := e.(*expression.ScalarFunction)
+		if !ok {
+			return nil
+		}
+		if f.FuncName.L == lowerFuncName {
+			return f
+		}
+		return nil
+	}
+	orFunc := isFunc(expr, ast.LogicOr)
+	if orFunc == nil {
+		return false
+	}
+	for i := 0; i < 2; i++ {
+		andFunc := isFunc(orFunc.GetArgs()[i], ast.LogicAnd)
+		if andFunc == nil {
+			continue
+		}
+		if !isNullRejected(ctx, schema, orFunc.GetArgs()[1-i]) {
+			continue // the other side should be null-rejected: null-rejected OR (... AND ...)
+		}
+		for _, andItem := range expression.FlattenCNFConditions(andFunc) {
+			if isNullRejected(ctx, schema, andItem) {
+				return true // hit the case in the comment: null-rejected OR (null-rejected AND ...)
+			}
+		}
+	}
+	return false
+}
+
 // PredicatePushDown implements LogicalPlan PredicatePushDown interface.
 func (p *LogicalExpand) PredicatePushDown(predicates []expression.Expression, opt *logicalOptimizeOp) (ret []expression.Expression, retPlan LogicalPlan) {
 	// Note that, grouping column related predicates can't be pushed down, since grouping column has nullability change after Expand OP itself.