Fixed LP bug #776295.

If the value of the flag cond_false of an Item_equal object is
true then the print method must return the string '0'.

mysql-test/r/explain.result

@@ -298,3 +298,18 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 DEALLOCATE PREPARE stmt;
 DROP TABLE t1;
 End of 5.1 tests.
+#
+# Bug#776295: EXPLAIN EXTENDED with always false multiple equality
+#             in the WHERE condition of a derived table
+#
+CREATE TABLE t1 (a int) ;
+CREATE TABLE t2 (a int) ;
+INSERT INTO t2 VALUES (8);
+EXPLAIN EXTENDED
+SELECT * FROM ( SELECT t1.a FROM t1,t2 WHERE t2.a = t1.a ) AS t;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	<derived2>	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
+2	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+Warnings:
+Note	1003	select NULL AS `a` from (select NULL AS `a` from `test`.`t1` join `test`.`t2` where 0) `t`
+DROP TABLE t1,t2;

mysql-test/r/join_outer.result

@@ -1302,7 +1302,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 1	SIMPLE	t2	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select '1' AS `f1`,NULL AS `f2`,'3' AS `f3`,NULL AS `f1`,NULL AS `f2` from `test`.`t1` left join `test`.`t2` on(multiple equal(NULL)) where ((coalesce('1',NULL),'3') in ((1,3),(2,2)))
+Note	1003	select '1' AS `f1`,NULL AS `f2`,'3' AS `f3`,NULL AS `f1`,NULL AS `f2` from `test`.`t1` left join `test`.`t2` on(0) where ((coalesce('1',NULL),'3') in ((1,3),(2,2)))
 SELECT * FROM t1 LEFT JOIN t2 ON t1.f2 = t2.f2
 WHERE (COALESCE(t1.f1, t2.f1), f3) IN ((1, 3), (2, 2));
 f1	f2	f3	f1	f2

mysql-test/r/join_outer_jcl6.result

@@ -1309,7 +1309,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 1	SIMPLE	t2	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select '1' AS `f1`,NULL AS `f2`,'3' AS `f3`,NULL AS `f1`,NULL AS `f2` from `test`.`t1` left join `test`.`t2` on(multiple equal(NULL)) where ((coalesce('1',NULL),'3') in ((1,3),(2,2)))
+Note	1003	select '1' AS `f1`,NULL AS `f2`,'3' AS `f3`,NULL AS `f1`,NULL AS `f2` from `test`.`t1` left join `test`.`t2` on(0) where ((coalesce('1',NULL),'3') in ((1,3),(2,2)))
 SELECT * FROM t1 LEFT JOIN t2 ON t1.f2 = t2.f2
 WHERE (COALESCE(t1.f1, t2.f1), f3) IN ((1, 3), (2, 2));
 f1	f2	f3	f1	f2

mysql-test/t/explain.test

@@ -265,3 +265,19 @@ DEALLOCATE PREPARE stmt;
 DROP TABLE t1;
 
 --echo End of 5.1 tests.
+
+--echo #
+--echo # Bug#776295: EXPLAIN EXTENDED with always false multiple equality
+--echo #             in the WHERE condition of a derived table
+--echo #
+
+CREATE TABLE t1 (a int) ;
+
+CREATE TABLE t2 (a int) ;
+INSERT INTO t2 VALUES (8);
+
+EXPLAIN EXTENDED
+SELECT * FROM ( SELECT t1.a FROM t1,t2 WHERE t2.a = t1.a ) AS t;
+
+DROP TABLE t1,t2;
+

sql/item_cmpfunc.cc

@@ -5923,6 +5923,11 @@ Item *Item_equal::transform(Item_transformer transformer, uchar *arg)
 
 void Item_equal::print(String *str, enum_query_type query_type)
 {
+  if (cond_false)
+  {
+    str->append('0');
+    return;
+  }
   str->append(func_name());
   str->append('(');
   List_iterator_fast<Item> it(equal_items);