- a fix for Bug#11458 "Prepared statement with subselects return random

data": remove the fix for another bug (8807) that
added OUTER_REF_TABLE_BIT to all subqueries that used a placeholder
to prevent their evaluation at prepare. As this bit hanged in 
Item_subselect::used_tables_cache for ever, a constant subquery with
a placeholder was never evaluated as such, which caused wrong 
choice of the execution plan for the statement.
- to fix Bug#8807 backport a better fix from 5.0
- post-review fixes.

sql/sql_parse.cc

@@ -4123,6 +4123,14 @@ mysql_new_select(LEX *lex, bool move_down)
   select_lex->select_number= ++lex->thd->select_number;
   select_lex->init_query();
   select_lex->init_select();
+  /*
+    Don't evaluate this subquery during statement prepare even if
+    it's a constant one. The flag is switched off in the end of
+    mysql_stmt_prepare.
+  */
+  if (lex->thd->current_arena->is_stmt_prepare())
+    select_lex->uncacheable|= UNCACHEABLE_PREPARE;
+
   if (move_down)
   {
     lex->subqueries= TRUE;