fixed uncachable union (bug #150)

mysql-test/r/union.result

@@ -269,3 +269,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 drop table t1,t2;
 (select 1) union (select 2) order by 0;
 Unknown column '0' in 'order clause'
+SELECT @a:=1 UNION SELECT @a:=@a+1;
+@a:=1
+1
+2

mysql-test/t/union.test

@@ -144,3 +144,5 @@ explain (select * from t1 where a=1) union (select * from t1 where b=1);
 drop table t1,t2;
 --error 1054
 (select 1) union (select 2) order by 0;
+
+SELECT @a:=1 UNION SELECT @a:=@a+1;

sql/sql_lex.h

@@ -501,11 +501,13 @@ typedef struct st_lex
       but we should merk all subselects as uncacheable from current till
       most upper
     */
-    for (SELECT_LEX_NODE *sl= current_select;
-	 sl != &select_lex;
-	 sl= sl->outer_select())
+    SELECT_LEX_NODE *sl;
+    SELECT_LEX_UNIT *un;
+    for (sl= current_select, un= sl->master_unit();
+	 un != &unit;
+	 sl= sl->outer_select(), un= sl->master_unit())
     {
-      sl->uncacheable = sl->master_unit()->uncacheable= 1;
+      sl->uncacheable = un->uncacheable= 1;
     }
   }
 } LEX;