BUG#822134: Invalid plan and wrong result set for Q20 from DBT3 benchmark set

- create_ref_for_key() has the code that walks KEYUSE array and tries to use
  maximum number of keyparts for ref (and eq_ref and ref_or_null) access.
  When one constructs ref access for table that is inside a SJ-Materialization
  nest, it is not possible to use tables that are ouside the nest (because 
  materialization is performed before they have any "current value").
  The bug was caused by this function not taking this into account.

sql/sql_select.cc

@@ -7189,6 +7189,32 @@ static bool create_hj_key_for_table(JOIN *join, JOIN_TAB *join_tab,
   DBUG_RETURN(FALSE);
 }
 
+/* 
+  Check if a set of tables specified by used_tables can be accessed when
+  we're doing scan on join_tab jtab.
+*/
+static bool are_tables_local(JOIN_TAB *jtab, table_map used_tables)
+{
+  if (jtab->bush_root_tab)
+  {
+    /*
+      jtab is inside execution join nest. We may not refer to outside tables,
+      except the const tables.
+    */
+    table_map local_tables= jtab->emb_sj_nest->nested_join->used_tables |
+                            jtab->join->const_table_map;
+    return !test(used_tables & ~local_tables);
+  }
+
+  /* 
+    If we got here then jtab is at top level. 
+     - all other tables at top level are accessible,
+     - tables in join nests are accessible too, because all their columns that 
+       are needed at top level will be unpacked when scanning the
+       materialization table.
+  */
+  return TRUE;
+}
 
 static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
                                KEYUSE *org_keyuse, table_map used_tables)
@@ -7234,14 +7260,17 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
     {
       if (!(~used_tables & keyuse->used_tables))
       {
-        if ((is_hash_join_key_no(key) && 
-            (keyparts == 0 || keyuse->keypart != (keyuse-1)->keypart)) ||
-            (!is_hash_join_key_no(key) && keyparts == keyuse->keypart &&
-             !(found_part_ref_or_null & keyuse->optimize)))
+        if  (are_tables_local(j, keyuse->val->used_tables()))
         {
-           length+= keyinfo->key_part[keyparts].store_length;
-           keyparts++;
-           found_part_ref_or_null|= keyuse->optimize & ~KEY_OPTIMIZE_EQ;
+          if ((is_hash_join_key_no(key) && 
+              (keyparts == 0 || keyuse->keypart != (keyuse-1)->keypart)) ||
+              (!is_hash_join_key_no(key) && keyparts == keyuse->keypart &&
+               !(found_part_ref_or_null & keyuse->optimize)))
+          {
+             length+= keyinfo->key_part[keyparts].store_length;
+             keyparts++;
+             found_part_ref_or_null|= keyuse->optimize & ~KEY_OPTIMIZE_EQ;
+          }
         }
       }
       keyuse++;