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, 3 insertions, 4 deletions

diff --git a/sql/item_func.cc b/sql/item_func.cc
index b382dbf7bf9..bf85e5b378a 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -160,14 +160,13 @@ bool Item_func::agg_arg_charsets(DTCollation &coll,
   }
 
   THD *thd= current_thd;
-  Item_arena *arena= thd->current_arena, backup;
+  Item_arena *arena, backup;
   bool res= FALSE;
   /*
     In case we're in statement prepare, create conversion item
     in its memory: it will be reused on each execute.
   */
-  if (arena->is_stmt_prepare())
-    thd->set_n_backup_item_arena(arena, &backup);
+  arena= thd->change_arena_if_needed(&backup);
 
   for (arg= args, last= args + nargs; arg < last; arg++)
   {
@@ -193,7 +192,7 @@ bool Item_func::agg_arg_charsets(DTCollation &coll,
     conv->fix_fields(thd, 0, &conv);
     *arg= conv;
   }
-  if (arena->is_stmt_prepare())
+  if (arena)
     thd->restore_backup_item_arena(arena, &backup);
   return res;
 }