Some changes from 4.0.

Take a look their for details

mysql-test/r/union.result

@@ -77,6 +77,8 @@ a	b
 (select a,b from t1 limit 2)  union all (select a,b from t2 order by a limit 1);
 a	b
 1	a
+2	b
+3	c
 (select a,b from t1 limit 2)  union all (select a,b from t2 order by a limit 1) order by b desc;
 a	b
 3	c
@@ -157,3 +159,22 @@ testtt
 tsestset
 1
 drop table t1;
+drop table if exists t1,t2;
+create table t1 (a int);
+create table t2 (a int);
+insert into t1 values (1),(2),(3),(4),(5);
+insert into t2 values (11),(12),(13),(14),(15);
+(select * from t1 limit 2) union (select * from t2 limit 3) limit 4;
+a
+1
+2
+11
+12
+(select * from t1 limit 2) union (select * from t2 limit 3);
+a
+1
+2
+11
+12
+13
+drop table t1,t2;

mysql-test/t/union.test

@@ -77,3 +77,11 @@ SELECT pseudo1 FROM t1 WHERE pseudo='joce' UNION SELECT pseudo FROM t1 WHERE pse
 SELECT pseudo1 FROM t1 WHERE pseudo='joce' UNION ALL SELECT pseudo FROM t1 WHERE pseudo1='joce';
 SELECT pseudo1 FROM t1 WHERE pseudo='joce' UNION SELECT 1;
 drop table t1;
+drop table if exists t1,t2;
+create table t1 (a int);
+create table t2 (a int);
+insert into t1 values (1),(2),(3),(4),(5);
+insert into t2 values (11),(12),(13),(14),(15);
+(select * from t1 limit 2) union (select * from t2 limit 3) limit 4;
+(select * from t1 limit 2) union (select * from t2 limit 3);
+drop table t1,t2;

sql/sql_union.cc

@@ -184,6 +184,8 @@ int mysql_union(THD *thd, LEX *lex,select_result *result)
 	if (thd->select_limit == HA_POS_ERROR)
 	  thd->options&= ~OPTION_FOUND_ROWS;
       }
+      else 
+	thd->select_limit= HA_POS_ERROR;		// no limit
       if (describe)
 	thd->select_limit= HA_POS_ERROR;		// no limit
       res=mysql_select(thd,&result_table_list,

sql/sql_yacc.yy

@@ -1441,22 +1441,22 @@ select_option_list:
 
 select_option:
 	STRAIGHT_JOIN { Select->options|= SELECT_STRAIGHT_JOIN; }
-	| HIGH_PRIORITY { Lex->lock_option= TL_READ_HIGH_PRIORITY; }
+	| HIGH_PRIORITY { if (Select != &Lex->select_lex) YYABORT;  Lex->lock_option= TL_READ_HIGH_PRIORITY; }
 	| DISTINCT	{ Select->options|= SELECT_DISTINCT; }
 	| SQL_SMALL_RESULT { Select->options|= SELECT_SMALL_RESULT; }
 	| SQL_BIG_RESULT { Select->options|= SELECT_BIG_RESULT; }
-	| SQL_BUFFER_RESULT { Select->options|= OPTION_BUFFER_RESULT; }
-	| SQL_CALC_FOUND_ROWS { Select->options|= OPTION_FOUND_ROWS; }
-	| SQL_NO_CACHE_SYM { current_thd->safe_to_cache_query=0; }
-	| SQL_CACHE_SYM { Select->options |= OPTION_TO_QUERY_CACHE; }
+	| SQL_BUFFER_RESULT { if (Select != &Lex->select_lex) YYABORT; Select->options|= OPTION_BUFFER_RESULT; }
+	| SQL_CALC_FOUND_ROWS {  if (Select != &Lex->select_lex) YYABORT; Select->options|= OPTION_FOUND_ROWS; }
+	| SQL_NO_CACHE_SYM {  if (Select != &Lex->select_lex) YYABORT; current_thd->safe_to_cache_query=0; }
+	| SQL_CACHE_SYM {  if (Select != &Lex->select_lex) YYABORT; Select->options |= OPTION_TO_QUERY_CACHE; }
 	| ALL		{}
 
 select_lock_type:
 	/* empty */
 	| FOR_SYM UPDATE_SYM
-	  { Lex->lock_option= TL_WRITE; current_thd->safe_to_cache_query=0; }
+	  {  if (Select != &Lex->select_lex) YYABORT;  Lex->lock_option= TL_WRITE; current_thd->safe_to_cache_query=0; }
 	| LOCK_SYM IN_SYM SHARE_SYM MODE_SYM
-	  { Lex->lock_option= TL_READ_WITH_SHARED_LOCKS; current_thd->safe_to_cache_query=0; }
+	  {  if (Select != &Lex->select_lex) YYABORT;  Lex->lock_option= TL_READ_WITH_SHARED_LOCKS; current_thd->safe_to_cache_query=0; }
 
 select_item_list:
 	  select_item_list ',' select_item