Simpler arena swapping code

Now thd->mem_root is a pointer to thd->main_mem_root and THR_MALLOC is a pointer to thd->mem_root.
This gives us the following benefits:
- Allow us to easily detect if arena has already been swapped before (this fixes a bug in setup_conds() where arena was swaped twice in some cases)
- Faster swaps of arenas (as we don't have to copy the whole MEM_ROOT)
- We don't anymore have to call my_pthread_setspecific_ptr(THR_MALLOC,...) to change where memory is alloced. Now it's enough to set thd->mem_root

1 files changed, 7 insertions, 7 deletions

diff --git a/sql/sql_union.cc b/sql/sql_union.cc
index 0be554f58a2..e0e8f8d42c5 100644
--- a/sql/sql_union.cc
+++ b/sql/sql_union.cc
@@ -313,24 +313,24 @@ int st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
         We're in statement prepare or in execution
         of a conventional statement.
       */
-      Item_arena backup;
-      if (arena->is_stmt_prepare())
-	thd->set_n_backup_item_arena(arena, &backup);
+      Item_arena *tmp_arena,backup;
+      tmp_arena= thd->change_arena_if_needed(&backup);
+
       Field **field;
       for (field= table->field; *field; field++)
       {
 	Item_field *item= new Item_field(*field);
 	if (!item || item_list.push_back(item))
 	{
-          if (arena->is_stmt_prepare())
-	    thd->restore_backup_item_arena(arena, &backup);
+          if (tmp_arena)
+	    thd->restore_backup_item_arena(tmp_arena, &backup);
 	  DBUG_RETURN(-1);
 	}
       }
+      if (tmp_arena)
+        thd->restore_backup_item_arena(tmp_arena, &backup);
       if (arena->is_stmt_prepare())
       {
-	thd->restore_backup_item_arena(arena, &backup);
-
 	/* prepare fake select to initialize it correctly */
 	ulong options_tmp= init_prepare_fake_select_lex(thd);
 	if (!(fake_select_lex->join= new JOIN(thd, item_list, thd->options,