Fixed LP BUG#747278

The problem was that when we have single row subquery with no rows
Item_cache(es) which represent result row was not null and being
requested via element_index() returned random value.

The fix is setting all Item_cache(es) in NULL before executing the
query (reset() method) which guaranty NULL value of whole query
or its elements requested in any way if no rows was found.

set_null() method was added to Item_cache to guaranty correct NULL
value in case of reseting the cache.

1 files changed, 28 insertions, 0 deletions

diff --git a/sql/item.cc b/sql/item.cc
index d321b74c1fc..9ba93780334 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -8201,6 +8201,20 @@ void Item_cache::print(String *str, enum_query_type query_type)
   str->append(')');
 }
 
+/**
+  Assign to this cache NULL value if it is possible
+*/
+
+void Item_cache::set_null()
+{
+  if (maybe_null)
+  {
+    null_value= TRUE;
+    value_cached= TRUE;
+  }
+}
+
+
 bool  Item_cache_int::cache_value()
 {
   if (!example)
@@ -8671,6 +8685,20 @@ void Item_cache_row::bring_value()
 }
 
 
+/**
+  Assign to this cache NULL value if it is possible
+*/
+
+void Item_cache_row::set_null()
+{
+  Item_cache::set_null();
+  if (!values)
+    return;
+  for (uint i= 0; i < item_count; i++)
+    values[i]->set_null();
+};
+
+
 Item_type_holder::Item_type_holder(THD *thd, Item *item)
   :Item(thd, item), enum_set_typelib(0), fld_type(get_real_type(item))
 {