planner: revise isnullRejected check for And and OR

expression/expression.go

@@ -818,6 +818,10 @@ func EvaluateExprWithNull(ctx sessionctx.Context, schema *Schema, expr Expressio
 	if MaybeOverOptimized4PlanCache(ctx, []Expression{expr}) {
 		return expr
 	}
+	if ctx.GetSessionVars().StmtCtx.InNullRejectCheck {
+		expr, _ = evaluateExprWithNullInNullRejectCheck(ctx, schema, expr)
+		return expr
+	}
 	return evaluateExprWithNull(ctx, schema, expr)
 }
 
@@ -842,6 +846,70 @@ func evaluateExprWithNull(ctx sessionctx.Context, schema *Schema, expr Expressio
 	return expr
 }
 
+// evaluateExprWithNullInNullRejectCheck sets columns in schema as null and calculate the final result of the scalar function.
+// If the Expression is a non-constant value, it means the result is unknown.
+// The returned bool values indicates whether the value is influenced by the Null Constant transformed from schema column
+// when the value is Null Constant.
+func evaluateExprWithNullInNullRejectCheck(ctx sessionctx.Context, schema *Schema, expr Expression) (Expression, bool) {
+	switch x := expr.(type) {
+	case *ScalarFunction:
+		args := make([]Expression, len(x.GetArgs()))
+		nullFromSets := make([]bool, len(x.GetArgs()))
+		for i, arg := range x.GetArgs() {
+			args[i], nullFromSets[i] = evaluateExprWithNullInNullRejectCheck(ctx, schema, arg)
+		}
+
+		// allNullFromSet indicates whether all arguments are Null Constant and the Null Constant is affected by the column of the schema.
+		allNullFromSet := true
+		for i := range args {
+			if cons, ok := args[i].(*Constant); ok && cons.Value.IsNull() && !nullFromSets[i] {
+				allNullFromSet = false
+				break
+			}
+		}
+
+		// allArgsNullFromSet indicates whether all Null Constant are affected by the column of the schema
+		allArgsNullFromSet := true
+		for i := range args {
+			if cons, ok := args[i].(*Constant); ok && cons.Value.IsNull() && nullFromSets[i] {
+				continue
+			}
+			allArgsNullFromSet = false
+		}
+
+		// If all the args are Null Constant and affected by the column schema, then we should keep it.
+		// Otherwise, we shouldn't let Null Constant which affected by the column schema participate in computing in `And` and `OR`
+		// due to the result of `AND` and `OR are uncertain if one of the arguments is NULL.
+		if !allArgsNullFromSet {
+			for i := range args {
+				if cons, ok := args[i].(*Constant); ok && cons.Value.IsNull() && nullFromSets[i] {
+					if x.FuncName.L == ast.LogicAnd {
+						args[i] = NewOne()
+					}
+					if x.FuncName.L == ast.LogicOr {
+						args[i] = NewZero()
+					}
+				}
+			}
+		}
+		c := NewFunctionInternal(ctx, x.FuncName.L, x.RetType.Clone(), args...)
+		cons, ok := c.(*Constant)
+		// If the return expr is Null Constant, and all the Null Constant arguments are affected by column schema,
+		// then we think the result Null Constant is also affected by the column schema
+		return c, ok && cons.Value.IsNull() && allNullFromSet
+	case *Column:
+		if !schema.Contains(x) {
+			return x, false
+		}
+		return &Constant{Value: types.Datum{}, RetType: types.NewFieldType(mysql.TypeNull)}, true
+	case *Constant:
+		if x.DeferredExpr != nil {
+			return FoldConstant(x), false
+		}
+	}
+	return expr, false
+}
+
 // TableInfo2SchemaAndNames converts the TableInfo to the schema and name slice.
 func TableInfo2SchemaAndNames(ctx sessionctx.Context, dbName model.CIStr, tbl *model.TableInfo) (*Schema, []*types.FieldName, error) {
 	cols, names, err := ColumnInfos2ColumnsAndNames(ctx, dbName, tbl.Name, tbl.Cols(), tbl)

planner/core/plan_test.go

@@ -1076,3 +1076,19 @@ func TestNullEQConditionPlan(t *testing.T) {
 			{"Point_Get_5", "root", "handle:0"},
 		})
 }
+
+// https://github.com/pingcap/tidb/issues/38304
+func TestOuterJoinOnNull(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("CREATE TABLE t0(c0 BLOB(5), c1 BLOB(5));")
+	tk.MustExec("CREATE TABLE t1 (c0 BOOL);")
+	tk.MustExec("INSERT INTO t1 VALUES(false);")
+	tk.MustExec("INSERT INTO t0(c0, c1) VALUES ('>', true);")
+	tk.MustQuery("SELECT * FROM t0 LEFT OUTER JOIN t1 ON NULL; ").Check(testkit.Rows("> 1 <nil>"))
+	tk.MustQuery("SELECT NOT '2' =(t1.c0 AND t0.c1 IS NULL) FROM t0 LEFT OUTER JOIN t1 ON NULL; ").Check(testkit.Rows("1"))
+	tk.MustQuery("SELECT * FROM t0 LEFT JOIN t1 ON NULL WHERE NOT '2' =(t1.c0 AND t0.c1 IS NULL); ").Check(testkit.Rows("> 1 <nil>"))
+	tk.MustQuery("SELECT * FROM t0 LEFT JOIN t1 ON NULL WHERE t1.c0 or true; ").Check(testkit.Rows("> 1 <nil>"))
+	tk.MustQuery("SELECT * FROM t0 LEFT JOIN t1 ON NULL WHERE not(t1.c0 and false); ").Check(testkit.Rows("> 1 <nil>"))
+}

planner/core/rule_predicate_push_down.go

@@ -440,16 +440,20 @@ func isNullRejected(ctx sessionctx.Context, schema *expression.Schema, expr expr
 	}
 	sc := ctx.GetSessionVars().StmtCtx
 	sc.InNullRejectCheck = true
-	result := expression.EvaluateExprWithNull(ctx, schema, expr)
-	sc.InNullRejectCheck = false
-	x, ok := result.(*expression.Constant)
-	if !ok {
-		return false
-	}
-	if x.Value.IsNull() {
-		return true
-	} else if isTrue, err := x.Value.ToBool(sc); err == nil && isTrue == 0 {
-		return true
+	defer func() {
+		sc.InNullRejectCheck = false
+	}()
+	for _, cond := range expression.SplitCNFItems(expr) {
+		result := expression.EvaluateExprWithNull(ctx, schema, cond)
+		x, ok := result.(*expression.Constant)
+		if !ok {
+			continue
+		}
+		if x.Value.IsNull() {
+			return true
+		} else if isTrue, err := x.Value.ToBool(sc); err == nil && isTrue == 0 {
+			return true
+		}
 	}
 	return false
 }