Fix for bug #12728: Very strange behaviour of ELT

mysql-test/r/func_str.result:
  Fix for bug #12728: Very strange behaviour of ELT
    - test case
mysql-test/t/func_str.test:
  Fix for bug #12728: Very strange behaviour of ELT
    - test result
sql/item_strfunc.cc:
  Fix for bug #12728: Very strange behaviour of ELT
    - Item_func_elt::eq() introduced: check 'item' member as well
      (to distinguish for instance elt(1, 'a', 'b') and elt(2, 'a', 'b')
sql/item_strfunc.h:
  Fix for bug #12728: Very strange behaviour of ELT
    - Item_func_elt::eq() introduced: check 'item' member as well
      (to distinguish for instance elt(1, 'a', 'b') and elt(2, 'a', 'b')

mysql-test/r/func_str.result

@@ -436,3 +436,10 @@ id	aes_decrypt(str, 'bar')
 1	foo
 2	NULL
 DROP TABLE t1, t2;
+create table t1(a varchar(8), primary key(a));
+insert into t1 values('bar'), ('foo');
+select a from t1 where a=elt(1, 'foo', 'bar') or a=elt(2, 'foo', 'bar');
+a
+bar
+foo
+drop table t1;

mysql-test/t/func_str.test

@@ -254,3 +254,11 @@ SELECT t1.id, aes_decrypt(str, 'bar') FROM t1, t2 WHERE t1.id = t2.id
 
 DROP TABLE t1, t2;
 
+#
+# Bug #12728: strange elt() behavior
+#
+
+create table t1(a varchar(8), primary key(a));
+insert into t1 values('bar'), ('foo');
+select a from t1 where a=elt(1, 'foo', 'bar') or a=elt(2, 'foo', 'bar');
+drop table t1;

sql/item_strfunc.cc

@@ -1599,6 +1599,17 @@ String *Item_func_elt::val_str(String *str)
 }
 
 
+bool Item_func_elt::eq(const Item *par_item, bool binary_cmp) const
+{
+  /* 
+    We can use (Item_func_elt*) typecast here because the check is done 
+    in the Item_func::eq().
+  */
+  return Item_func::eq(par_item, binary_cmp) &&
+         item->eq(((Item_func_elt*) par_item)->item, binary_cmp);
+}
+
+
 void Item_func_make_set::split_sum_func(List<Item> &fields)
 {
   if (item->with_sum_func && item->type() != SUM_FUNC_ITEM)

sql/item_strfunc.h

@@ -372,6 +372,7 @@ public:
   void fix_length_and_dec();
   void update_used_tables();
   const char *func_name() const { return "elt"; }
+  bool eq(const Item *par_item, bool binary_cmp) const;
   unsigned int size_of() { return sizeof(*this);}  
 };