Bug #32036: EXISTS within a WHERE clause with a UNION

  crashes MySQL 5.122
There was a difference in how UNIONs are handled
on top level and when in sub-query.
Because the rules for sub-queries were syntactically
allowing cases that are not currently supported by
the server we had crashes (this bug) or wrong results
(bug 32051).
Fixed by making the syntax rules for UNIONs match the 
ones at top level.

These rules however do not support nesting UNIONs, e.g.
(SELECT a FROM t1 UNION ALL SELECT b FROM t2) 
 UNION
(SELECT c FROM t3 UNION ALL SELECT d FROM t4)
Supports for statements with nested UNIONs will be
added in a future version.

mysql-test/r/subselect.result

@@ -3558,22 +3558,19 @@ SELECT sql_no_cache * FROM t1 WHERE NOT EXISTS
 (SELECT i FROM t1) UNION 
 (SELECT i FROM t1)
 );
-i
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION 
+(SELECT i FROM t1)
+)' at line 3
 SELECT * FROM t1 
 WHERE NOT EXISTS (((SELECT i FROM t1) UNION (SELECT i FROM t1)));
-i
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION (SELECT i FROM t1)))' at line 2
 explain select ((select t11.i from t1 t11) union (select t12.i from t1 t12))
 from t1;
 ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'union (select t12.i from t1 t12))
 from t1' at line 1
 explain select * from t1 where not exists 
 ((select t11.i from t1 t11) union (select t12.i from t1 t12));
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	0	const row not found
-2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
-3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
-4	UNION	t12	system	NULL	NULL	NULL	NULL	0	const row not found
-NULL	UNION RESULT	<union2,4>	ALL	NULL	NULL	NULL	NULL	NULL	
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'union (select t12.i from t1 t12))' at line 2
 DROP TABLE t1;
 CREATE TABLE t1 (a VARCHAR(250), b INT auto_increment, PRIMARY KEY (b));
 insert into t1 (a) values (FLOOR(rand() * 100));
@@ -4150,4 +4147,26 @@ SELECT ((a1,a2) IN (SELECT * FROM t2 WHERE b2 > 0)) IS NULL FROM t1;
 0
 0
 DROP TABLE t1, t2;
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+INSERT INTO t2 VALUES (1),(2);
+SELECT 2 FROM t1 WHERE EXISTS ((SELECT 1 FROM t2 WHERE t1.a=t2.a));
+2
+2
+2
+EXPLAIN EXTENDED
+SELECT 2 FROM t1 WHERE EXISTS ((SELECT 1 FROM t2 WHERE t1.a=t2.a));
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+Warnings:
+Note	1276	Field or reference 'test.t1.a' of SELECT #2 was resolved in SELECT #1
+Note	1003	select 2 AS `2` from `test`.`t1` where exists(select 1 AS `1` from `test`.`t2` where (`test`.`t1`.`a` = `test`.`t2`.`a`))
+EXPLAIN EXTENDED
+SELECT 2 FROM t1 WHERE EXISTS ((SELECT 1 FROM t2 WHERE t1.a=t2.a) UNION 
+(SELECT 1 FROM t2 WHERE t1.a = t2.a));
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION 
+(SELECT 1 FROM t2 WHERE t1.a = t2.a))' at line 2
+DROP TABLE t1,t2;
 End of 5.0 tests.

mysql-test/t/subselect.test

@@ -2448,12 +2448,16 @@ DROP TABLE t1, t2;
 CREATE TABLE t1 (i INT);
 
 (SELECT i FROM t1) UNION (SELECT i FROM t1);
+#TODO:not supported
+--error ER_PARSE_ERROR
 SELECT sql_no_cache * FROM t1 WHERE NOT EXISTS 
   (
    (SELECT i FROM t1) UNION 
    (SELECT i FROM t1)
   );
 
+#TODO:not supported
+--error ER_PARSE_ERROR
 SELECT * FROM t1 
 WHERE NOT EXISTS (((SELECT i FROM t1) UNION (SELECT i FROM t1)));
 
@@ -2461,7 +2465,9 @@ WHERE NOT EXISTS (((SELECT i FROM t1) UNION (SELECT i FROM t1)));
 --error 1064
 explain select ((select t11.i from t1 t11) union (select t12.i from t1 t12))
   from t1;
-#supported
+
+#TODO:not supported
+--error ER_PARSE_ERROR
 explain select * from t1 where not exists 
   ((select t11.i from t1 t11) union (select t12.i from t1 t12));
 
@@ -3002,4 +3008,27 @@ INSERT INTO t2 VALUES (103, 203);
 SELECT ((a1,a2) IN (SELECT * FROM t2 WHERE b2 > 0)) IS NULL FROM t1;
 DROP TABLE t1, t2;
 
+#
+# Bug #32036: EXISTS within a WHERE clause with a UNION crashes MySQL 5.122
+#
+
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (a INT);
+
+INSERT INTO t1 VALUES (1),(2);
+INSERT INTO t2 VALUES (1),(2);
+
+SELECT 2 FROM t1 WHERE EXISTS ((SELECT 1 FROM t2 WHERE t1.a=t2.a));
+EXPLAIN EXTENDED
+SELECT 2 FROM t1 WHERE EXISTS ((SELECT 1 FROM t2 WHERE t1.a=t2.a));
+
+
+#TODO:not supported
+--error ER_PARSE_ERROR
+EXPLAIN EXTENDED
+SELECT 2 FROM t1 WHERE EXISTS ((SELECT 1 FROM t2 WHERE t1.a=t2.a) UNION 
+                               (SELECT 1 FROM t2 WHERE t1.a = t2.a));
+
+DROP TABLE t1,t2;
+
 --echo End of 5.0 tests.

sql/sql_yacc.yy

@@ -1137,7 +1137,7 @@ bool my_yyoverflow(short **a, YYSTYPE **b, ulong *yystacksize);
 
 %type <variable> internal_variable_name
 
-%type <select_lex> subselect subselect_init
+%type <select_lex> subselect take_first_select
 	get_select_lex
 
 %type <boolfunc2creator> comp_op
@@ -9422,34 +9422,21 @@ union_option:
 	| ALL       { $$=0; }
         ;
 
+take_first_select: /* empty */
+        {
+          $$= Lex->current_select->master_unit()->first_select();
+        };
+
 subselect:
-        SELECT_SYM subselect_start subselect_init subselect_end
+        SELECT_SYM subselect_start select_init2 take_first_select 
+        subselect_end
         {
-          $$= $3;
+          $$= $4;
         }
-        | '(' subselect_start subselect ')'
+        | '(' subselect_start select_paren take_first_select 
+        subselect_end ')'
         {
-	    THD *thd= YYTHD;
-            /*
-              note that a local variable can't be used for
-              $3 as it's used in local variable construction
-              and some compilers can't guarnatee the order
-              in which the local variables are initialized.
-            */
-            List_iterator<Item> it($3->item_list);
-            Item *item;
-            /*
-              we must fill the items list for the "derived table".
-            */
-            while ((item= it++))
-              add_item_to_list(thd, item);
-          }
-          union_clause subselect_end { $$= $3; };
-
-subselect_init:
-  select_init2
-  {
-    $$= Lex->current_select->master_unit()->first_select();
+          $$= $4;
         };
 
 subselect_start: