Fixed bug #26017.

Objects of the class Item_equal contain an auxiliary member
eval_item of the type cmp_item that is used only for direct 
evaluation of multiple equalities. Currently a multiple equality
is evaluated directly only in the cases when the equality holds
at most for one row in the result set.
The compare collation of eval_item was determined incorectly.
It could lead to returning incorrect results for some queries.

mysql-test/r/join_outer.result

@@ -1194,3 +1194,23 @@ a	b
 3	3
 4	NULL
 DROP TABLE t1,t2;
+CREATE TABLE t1 (
+f1 varchar(16) collate latin1_swedish_ci PRIMARY KEY,
+f2 varchar(16) collate latin1_swedish_ci
+);
+CREATE TABLE t2 (
+f1 varchar(16) collate latin1_swedish_ci PRIMARY KEY,
+f3 varchar(16) collate latin1_swedish_ci
+);
+INSERT INTO t1 VALUES ('bla','blah');
+INSERT INTO t2 VALUES ('bla','sheep');
+SELECT * FROM t1 JOIN t2 USING(f1) WHERE f1='Bla';
+f1	f2	f3
+bla	blah	sheep
+SELECT * FROM t1 LEFT JOIN t2 USING(f1) WHERE f1='bla';
+f1	f2	f3
+bla	blah	sheep
+SELECT * FROM t1 LEFT JOIN t2 USING(f1) WHERE f1='Bla';
+f1	f2	f3
+bla	blah	sheep
+DROP TABLE t1,t2;

mysql-test/t/join_outer.test

@@ -802,3 +802,26 @@ SELECT * FROM t1 LEFT JOIN t2 ON t1.a = t2.b WHERE (1=1 OR 2=2);
 SELECT * FROM t1 LEFT JOIN t2 ON t1.a = t2.b WHERE (1=1 OR 1=0);
 
 DROP TABLE t1,t2;
+
+#
+# Bug 26017: LEFT OUTER JOIN over two constant tables and 
+#            a case-insensitive comparison predicate field=const 
+# 
+
+CREATE TABLE t1 (
+  f1 varchar(16) collate latin1_swedish_ci PRIMARY KEY,
+  f2 varchar(16) collate latin1_swedish_ci
+);
+CREATE TABLE t2 (
+  f1 varchar(16) collate latin1_swedish_ci PRIMARY KEY,
+  f3 varchar(16) collate latin1_swedish_ci
+);
+
+INSERT INTO t1 VALUES ('bla','blah');
+INSERT INTO t2 VALUES ('bla','sheep');
+
+SELECT * FROM t1 JOIN t2 USING(f1) WHERE f1='Bla';
+SELECT * FROM t1 LEFT JOIN t2 USING(f1) WHERE f1='bla';
+SELECT * FROM t1 LEFT JOIN t2 USING(f1) WHERE f1='Bla';
+
+DROP TABLE t1,t2;

sql/item_cmpfunc.cc

@@ -4034,11 +4034,9 @@ longlong Item_equal::val_int()
 
 void Item_equal::fix_length_and_dec()
 {
-  Item *item= const_item ? const_item : get_first();
+  Item *item= get_first();
   eval_item= cmp_item::get_comparator(item->result_type(),
                                       item->collation.collation);
-  if (item->result_type() == STRING_RESULT)
-    eval_item->cmp_charset= cmp_collation.collation;
 }
 
 bool Item_equal::walk(Item_processor processor, byte *arg)

sql/item_cmpfunc.h

@@ -1296,7 +1296,6 @@ class Item_equal: public Item_bool_func
   Item *const_item;        /* optional constant item equal to fields items */
   cmp_item *eval_item;
   bool cond_false;
-  DTCollation cmp_collation;
 public:
   inline Item_equal()
     : Item_bool_func(), const_item(0), eval_item(0), cond_false(0)