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

…n constant folding (#52119)

close #51765

pkg/expression/constant_fold.go

@@ -100,7 +100,15 @@ func ifNullFoldHandler(ctx BuildContext, 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(ctx, args[1])
+			foldedExpr, isConstant := foldConstant(ctx, 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
 	}

tests/integrationtest/r/expression/constant_fold.result

@@ -0,0 +1,26 @@
+drop table if exists t, t1;
+create table t (id varbinary(16));
+create table t1(id char(16) charset utf8mb4 collate utf8mb4_general_ci);
+insert into t values ();
+insert into t1 values ("Hello World");
+select collation(ifnull(concat(NULL),'~'));
+collation(ifnull(concat(NULL),'~'))
+utf8mb4_general_ci
+select collation(ifnull(concat(NULL),ifnull(concat(NULL),'~')));
+collation(ifnull(concat(NULL),ifnull(concat(NULL),'~')))
+utf8mb4_general_ci
+select collation(ifnull(concat(id),'~')) from t;
+collation(ifnull(concat(id),'~'))
+binary
+select collation(ifnull(concat(NULL),ifnull(concat(id),'~'))) from t;
+collation(ifnull(concat(NULL),ifnull(concat(id),'~')))
+binary
+select collation(ifnull(concat(id),ifnull(concat(id),'~'))) from t;
+collation(ifnull(concat(id),ifnull(concat(id),'~')))
+binary
+select collation(ifnull(concat(NULL),id)) from t1;
+collation(ifnull(concat(NULL),id))
+utf8mb4_general_ci
+select collation(ifnull(concat(NULL),ifnull(concat(NULL),id))) from t1;
+collation(ifnull(concat(NULL),ifnull(concat(NULL),id)))
+utf8mb4_general_ci

tests/integrationtest/t/expression/constant_fold.test

@@ -0,0 +1,13 @@
+# TestFoldIfNull
+drop table if exists t, t1;
+create table t (id varbinary(16));
+create table t1(id char(16) charset utf8mb4 collate utf8mb4_general_ci);
+insert into t values ();
+insert into t1 values ("Hello World");
+select collation(ifnull(concat(NULL),'~'));
+select collation(ifnull(concat(NULL),ifnull(concat(NULL),'~')));
+select collation(ifnull(concat(id),'~')) from t;
+select collation(ifnull(concat(NULL),ifnull(concat(id),'~'))) from t;
+select collation(ifnull(concat(id),ifnull(concat(id),'~'))) from t;
+select collation(ifnull(concat(NULL),id)) from t1;
+select collation(ifnull(concat(NULL),ifnull(concat(NULL),id))) from t1;