Fixed the bug mdev-12788.

In some rare cases queries with UNION ALL
using a derived table specified by
a grouping select with a subquery in WHERE and
impossible HAVING detected after constant row
substitution could hang.
The cause was not a proper return from the
function subselect_single_select_engine::exec()
in the case when the subquery was not optimized
beforehand and the optimization performed
in this function requested for a change of the
subquery engine. This was fixed.
Also a change was applied that avoided execution
of a subquery if impossible having was detected
for the main query at the optimization stage.

mysql-test/r/union.result

@@ -2156,3 +2156,34 @@ WHERE t1_2.b NOT IN ( SELECT 4 UNION ALL SELECT 5 );
 a	b	a	b
 1	1	1	1
 DROP TABLE t1;
+#
+# Bug mdev-12788: UNION ALL + impossible having for derived
+#                 with IN subquery in WHERE
+#
+CREATE TABLE t1 (i int) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1);
+CREATE TABLE t2 (pk int PRIMARY KEY) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (1),(2);
+SELECT 1, 2
+UNION ALL
+SELECT i, COUNT(*) FROM (
+SELECT * FROM t1 WHERE i IN ( SELECT pk FROM t2 )
+) AS sq
+GROUP BY i
+HAVING i = 10;
+1	2
+1	2
+EXPLAIN EXTENDED SELECT 1, 2
+UNION ALL
+SELECT i, COUNT(*) FROM (
+SELECT * FROM t1 WHERE i IN ( SELECT pk FROM t2 )
+) AS sq
+GROUP BY i
+HAVING i = 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+2	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible HAVING noticed after reading const tables
+4	DEPENDENT SUBQUERY	t2	unique_subquery	PRIMARY	PRIMARY	4	func	1	100.00	Using index
+Warnings:
+Note	1003	select 1 AS `1`,2 AS `2` union all select 1 AS `i`,count(0) AS `COUNT(*)` from dual where <in_optimizer>(1,<exists>(<primary_index_lookup>(<cache>(1) in t2 on PRIMARY))) group by 1 having 0
+DROP TABLE t1,t2;

mysql-test/t/union.test

@@ -1507,3 +1507,28 @@ SELECT * FROM t1 t1_1 LEFT JOIN t1 t1_2 ON ( t1_2.b = t1_1.a )
   WHERE t1_2.b NOT IN ( SELECT 4 UNION ALL SELECT 5 );
 
 DROP TABLE t1;
+
+--echo #
+--echo # Bug mdev-12788: UNION ALL + impossible having for derived
+--echo #                 with IN subquery in WHERE
+--echo #
+
+CREATE TABLE t1 (i int) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1);
+
+CREATE TABLE t2 (pk int PRIMARY KEY) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (1),(2);
+
+let $q=
+SELECT 1, 2
+UNION ALL
+SELECT i, COUNT(*) FROM (
+  SELECT * FROM t1 WHERE i IN ( SELECT pk FROM t2 )
+) AS sq
+GROUP BY i
+HAVING i = 10;
+
+eval $q;
+eval EXPLAIN EXTENDED $q;
+
+DROP TABLE t1,t2;

sql/item_subselect.cc

@@ -3776,7 +3776,10 @@ int subselect_single_select_engine::exec()
       }
     }
     if (item->engine_changed(this))
+    {
+      thd->lex->current_select= save_select;
       DBUG_RETURN(1);
+    }
   }
   if (select_lex->uncacheable &&
       select_lex->uncacheable != UNCACHEABLE_EXPLAIN

sql/sql_select.cc

@@ -1984,7 +1984,8 @@ JOIN::optimize_inner()
       having= new (thd->mem_root) Item_int(thd, (longlong) 0,1);
       zero_result_cause= "Impossible HAVING noticed after reading const tables";
       error= 0;
-      DBUG_RETURN(0);
+      select_lex->mark_const_derived(zero_result_cause);
+      goto setup_subq_exit;
     }
   }
 
@@ -3377,7 +3378,8 @@ void JOIN::exec_inner()
     condtions may be arbitrarily costly, and because the optimize phase
     might not have produced a complete executable plan for EXPLAINs.
   */
-  if (exec_const_cond && !(select_options & SELECT_DESCRIBE) &&
+  if (!zero_result_cause &&
+      exec_const_cond && !(select_options & SELECT_DESCRIBE) &&
       !exec_const_cond->val_int())
     zero_result_cause= "Impossible WHERE noticed after reading const tables";