MDEV-25994: Crash with union of my_decimal type in ORDER BY clause

When single-row subquery fails with "Subquery reutrns more than 1 row"
error, it will raise an error and return NULL.

On the other hand, Item_singlerow_subselect sets item->maybe_null=0
for table-less subqueries like "(SELECT not_null_value)"  (*)

This discrepancy (item with maybe_null=0 returning NULL) causes the
code in Type_handler_decimal_result::make_sort_key_part() to crash.

Fixed this by allowing inference (*) only when the subquery is NOT a
UNION.

mysql-test/r/order_by.result

@@ -3508,4 +3508,24 @@ DELETE FROM t1 ORDER BY c;
 DROP TABLE t1;
 SET @@SESSION.max_sort_length=DEFAULT;
 SET sql_mode=DEFAULT;
+#
+# MDEV-25994 Crash with union of my_decimal type in ORDER BY clause
+#
+CREATE TABLE t1 (v1 INTEGER) ;
+INSERT INTO t1 (v1) VALUES (8);
+UPDATE t1 SET v1 = 1 ORDER BY (SELECT 1.1 UNION SELECT -1);
+ERROR 21000: Subquery returns more than 1 row
+# This one must be successful
+UPDATE t1 SET v1 = 2 ORDER BY (SELECT 1 UNION SELECT 1);
+UPDATE t1 SET v1 = 3 ORDER BY (SELECT 'a' UNION SELECT 'b');
+ERROR 21000: Subquery returns more than 1 row
+# Insert some more data
+INSERT INTO t1 (v1) VALUES (8),(9),(100),(-234),(46584),(0);
+UPDATE t1 SET v1 = v1+1 ORDER BY (SELECT 100.122 UNION SELECT -189.2);
+ERROR 21000: Subquery returns more than 1 row
+# This one must be successful
+UPDATE t1 SET v1 = v1-200 ORDER BY (SELECT 1 UNION SELECT 1);
+UPDATE t1 SET v1 = v1 ORDER BY (SELECT 'abc' UNION SELECT 'bbb');
+ERROR 21000: Subquery returns more than 1 row
+DROP TABLE t1;
 # End of 10.2 tests

mysql-test/r/subselect.result

@@ -1261,7 +1261,7 @@ a
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` bigint(20) NOT NULL
+  `a` bigint(20) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 create table t1 (a int);

mysql-test/r/subselect_no_exists_to_in.result

@@ -1265,7 +1265,7 @@ a
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` bigint(20) NOT NULL
+  `a` bigint(20) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 create table t1 (a int);

mysql-test/r/subselect_no_mat.result

@@ -1268,7 +1268,7 @@ a
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` bigint(20) NOT NULL
+  `a` bigint(20) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 create table t1 (a int);

mysql-test/r/subselect_no_opts.result

@@ -1264,7 +1264,7 @@ a
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` bigint(20) NOT NULL
+  `a` bigint(20) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 create table t1 (a int);

mysql-test/r/subselect_no_scache.result

@@ -1267,7 +1267,7 @@ a
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` bigint(20) NOT NULL
+  `a` bigint(20) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 create table t1 (a int);

mysql-test/r/subselect_no_semijoin.result

@@ -1264,7 +1264,7 @@ a
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `a` bigint(20) NOT NULL
+  `a` bigint(20) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 create table t1 (a int);

mysql-test/t/order_by.test

@@ -2331,5 +2331,30 @@ DROP TABLE t1;
 SET @@SESSION.max_sort_length=DEFAULT;
 SET sql_mode=DEFAULT;
 
+--echo #
+--echo # MDEV-25994 Crash with union of my_decimal type in ORDER BY clause
+--echo #
+
+CREATE TABLE t1 (v1 INTEGER) ;
+INSERT INTO t1 (v1) VALUES (8);
+--error ER_SUBQUERY_NO_1_ROW
+UPDATE t1 SET v1 = 1 ORDER BY (SELECT 1.1 UNION SELECT -1);
+--echo # This one must be successful
+UPDATE t1 SET v1 = 2 ORDER BY (SELECT 1 UNION SELECT 1);
+--error ER_SUBQUERY_NO_1_ROW
+UPDATE t1 SET v1 = 3 ORDER BY (SELECT 'a' UNION SELECT 'b');
+
+-- echo # Insert some more data
+INSERT INTO t1 (v1) VALUES (8),(9),(100),(-234),(46584),(0);
+--error ER_SUBQUERY_NO_1_ROW
+UPDATE t1 SET v1 = v1+1 ORDER BY (SELECT 100.122 UNION SELECT -189.2);
+--echo # This one must be successful
+UPDATE t1 SET v1 = v1-200 ORDER BY (SELECT 1 UNION SELECT 1);
+--error ER_SUBQUERY_NO_1_ROW
+UPDATE t1 SET v1 = v1 ORDER BY (SELECT 'abc' UNION SELECT 'bbb');
+
+
+DROP TABLE t1;
+
 
 --echo # End of 10.2 tests

sql/item_subselect.cc

@@ -1266,11 +1266,18 @@ bool Item_singlerow_subselect::fix_length_and_dec()
   }
   unsigned_flag= value->unsigned_flag;
   /*
-    If there are not tables in subquery then ability to have NULL value
-    depends on SELECT list (if single row subquery have tables then it
-    always can be NULL if there are not records fetched).
+    If the subquery has no tables (1) and is not a UNION (2), like:
+
+      (SELECT subq_value)
+
+    then its NULLability is the same as subq_value's NULLability.
+
+    (1): A subquery that uses a table will return NULL when the table is empty.
+    (2): A UNION subquery will return NULL if it produces a "Subquery returns
+         more than one row" error.
   */
-  if (engine->no_tables())
+  if (engine->no_tables() &&
+      engine->engine_type() != subselect_engine::UNION_ENGINE)
     maybe_null= engine->may_be_null();
   else
   {