expression: set a collation according to the arguments for ifnull i…

…n constant folding (#52119) (#52124)

close #51765

expression/constant_fold.go

@@ -100,7 +100,15 @@ func ifNullFoldHandler(expr *ScalarFunction) (Expression, bool) {
 		// evaluated to constArg.Value after foldConstant(args[0]), it's not
 		// needed to be checked.
 		if constArg.Value.IsNull() {
-			return foldConstant(args[1])
+			foldedExpr, isConstant := foldConstant(args[1])
+
+			// See https://github.com/pingcap/tidb/issues/51765. If the first argument can
+			// be folded into NULL, the collation of IFNULL should be the same as the second
+			// arguments.
+			expr.GetType().SetCharset(args[1].GetType().GetCharset())
+			expr.GetType().SetCollate(args[1].GetType().GetCollate())
+
+			return foldedExpr, isConstant
 		}
 		return constArg, isDeferred
 	}

expression/integration_test.go

@@ -8027,3 +8027,22 @@ func TestIssue50850(t *testing.T) {
 		testkit.Rows("01", "31", "34", "5D", "65", "A5", "A6", "B1", "D5", "FF"))
 	tk.MustExec("drop table if exists t3;")
 }
+
+func TestIssue51765(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+
+	tk.MustExec("use test")
+	tk.MustExec("create table t (id varbinary(16))")
+	tk.MustExec("create table t1(id char(16) charset utf8mb4 collate utf8mb4_general_ci)")
+	tk.MustExec("insert into t values ()")
+	tk.MustExec(`insert into t1 values ("Hello World")`)
+
+	tk.MustQuery("select collation(ifnull(concat(NULL), '~'))").Check(testkit.Rows("utf8mb4_bin"))
+	tk.MustQuery("select collation(ifnull(concat(NULL),ifnull(concat(NULL),'~')))").Check(testkit.Rows("utf8mb4_bin"))
+	tk.MustQuery("select collation(ifnull(concat(id),'~')) from t;").Check(testkit.Rows("binary"))
+	tk.MustQuery("select collation(ifnull(concat(NULL),ifnull(concat(id),'~'))) from t;").Check(testkit.Rows("binary"))
+	tk.MustQuery("select collation(ifnull(concat(id),ifnull(concat(id),'~'))) from t;").Check(testkit.Rows("binary"))
+	tk.MustQuery("select collation(ifnull(concat(NULL),id)) from t1;").Check(testkit.Rows("utf8mb4_general_ci"))
+	tk.MustQuery("select collation(ifnull(concat(NULL),ifnull(concat(NULL),id))) from t1;").Check(testkit.Rows("utf8mb4_general_ci"))
+}