expression: refactor constant folding for IF&IFNULL (#9094)

expression/constant_fold.go

@@ -41,7 +41,10 @@ func ifFoldHandler(expr *ScalarFunction) (Expression, bool) {
 	if constArg, isConst := foldedArg0.(*Constant); isConst {
 		arg0, isNull0, err := constArg.EvalInt(expr.Function.getCtx(), chunk.Row{})
 		if err != nil {
-			log.Warnf("fold constant %s: %s", expr.ExplainInfo(), err.Error())
+			// Failed to fold this expr to a constant, print the DEBUG log and
+			// return the original expression to let the error to be evaluated
+			// again, in that time, the error is returned to the client.
+			log.Debugf("fold constant %s: %s", expr.ExplainInfo(), err.Error())
 			return expr, false
 		}
 		if !isNull0 && arg0 != 0 {
@@ -59,16 +62,15 @@ func ifFoldHandler(expr *ScalarFunction) (Expression, bool) {
 
 func ifNullFoldHandler(expr *ScalarFunction) (Expression, bool) {
 	args := expr.GetArgs()
-	foldedArg0, _ := foldConstant(args[0])
+	foldedArg0, isDeferred := foldConstant(args[0])
 	if constArg, isConst := foldedArg0.(*Constant); isConst {
-		_, isNull0, err := constArg.EvalInt(expr.Function.getCtx(), chunk.Row{})
-		if err != nil {
-			log.Warnf("fold constant %s: %s", expr.ExplainInfo(), err.Error())
-			return expr, false
-		}
-		if isNull0 == true {
+		// Only check constArg.Value here. Because deferred expression is
+		// evaluated to constArg.Value after foldConstant(args[0]), it's not
+		// needed to be checked.
+		if constArg.Value.IsNull() {
 			return foldConstant(args[1])
 		}
+		return constArg, isDeferred
 	}
 	isDeferredConst := false
 	expr.GetArgs()[1], isDeferredConst = foldConstant(args[1])

expression/constant_fold_test.go

@@ -0,0 +1,35 @@
+// Copyright 2019 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package expression_test
+
+import (
+	. "github.com/pingcap/check"
+	"github.com/pingcap/tidb/util/testkit"
+)
+
+func (s *testIntegrationSuite) TestFoldIfNull(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec(`use test;`)
+	tk.MustExec(`drop table if exists t;`)
+	tk.MustExec(`create table t(a bigint, b bigint);`)
+	tk.MustExec(`insert into t values(1, 1);`)
+	tk.MustQuery(`desc select ifnull("aaaa", a) from t;`).Check(testkit.Rows(
+		`Projection_3 10000.00 root "aaaa"`,
+		`└─TableReader_5 10000.00 root data:TableScan_4`,
+		`  └─TableScan_4 10000.00 cop table:t, range:[-inf,+inf], keep order:false, stats:pseudo`,
+	))
+	tk.MustQuery(`show warnings;`).Check(testkit.Rows())
+	tk.MustQuery(`select ifnull("aaaa", a) from t;`).Check(testkit.Rows("aaaa"))
+	tk.MustQuery(`show warnings;`).Check(testkit.Rows())
+}