bugfix: multi-UPDATE, vcols, const tables

multi-update was setting up read_set/vcol_set in
multi_update::initialize_tables() that is invoked after
the optimizer (JOIN::optimize_inner()). But some rows - if they're from
const tables - will be read already in the optimizer, and these rows
will not have all necessary column/vcol values.

* multi_update::initialize_tables() uses results from the optimizer
  and cannot be moved to be called earlier.
* multi_update::prepare() is called before the optimizer, but
  it cannot set up read_set/vcol_set, because the optimizer
  might reset them (see SELECT_LEX::update_used_tables()).

As a fix I've added a new method, select_result::prepare_to_read_rows(),
it's called from inside the optimizer just before make_join_statistics().

1 files changed, 15 insertions, 2 deletions

diff --git a/sql/sql_update.cc b/sql/sql_update.cc
index da0e6cd2116..05f7080609a 100644
--- a/sql/sql_update.cc
+++ b/sql/sql_update.cc
@@ -1800,6 +1800,21 @@ void multi_update::update_used_tables()
   }
 }
 
+void multi_update::prepare_to_read_rows()
+{
+  /*
+    update column maps now. it cannot be done in ::prepare() before the
+    optimizer, because the optimize might reset them (in
+    SELECT_LEX::update_used_tables()), it cannot be done in
+    ::initialize_tables() after the optimizer, because the optimizer
+    might read rows from const tables
+  */
+
+  for (TABLE_LIST *tl= update_tables; tl; tl= tl->next_local)
+    tl->table->mark_columns_needed_for_update();
+}
+
+
 /*
   Check if table is safe to update on fly
 
@@ -1916,12 +1931,10 @@ multi_update::initialize_tables(JOIN *join)
     {
       if (safe_update_on_fly(thd, join->join_tab, table_ref, all_tables))
       {
-        table->mark_columns_needed_for_update();
 	table_to_update= table;			// Update table on the fly
 	continue;
       }
     }
-    table->mark_columns_needed_for_update();
     table->prepare_for_position();
 
     /*