Implementation of Monty's idea about clear_alloc_root() optimization and cleanup of work

with memory roots in THD/Statement/Item_arena.
Added assertions preventing memory allocation on bzero'ed MEM_ROOT since it is worked by 
pure luck and was very ineffective.


include/my_sys.h:
  Reimplementation of Monty's optimization of clear_alloc_root().
  Now clear_alloc_root() can be used only for detaching memory associated with MEM_ROOT
  (e.g. to avoid its freeing). It can not be used for MEM_ROOT initialization any longer 
  (it was bad idea anyway since memory allocation on such MEM_ROOT was very ineffective 
  and worked by pure luck).
  Introduced ALLOC_ROOT_MIN_BLOCK_SIZE constant.
mysys/my_alloc.c:
  Added description of init_alloc_root().
  Added assertions to alloc_root() and reset_root_defaults() so now they can only be used
  on previosly initialized MEM_ROOT. (It worked for bzeroed MEM_ROOT before but 
  by pure luck and very inefficiently). Calling free_root() on bzero'ed MEM_ROOT
  is still ok (we can't remove this easily because of static MEM_ROOTs).
  Also now using ALLOC_ROOT_MIN_BLOCK_SIZE constant inside these functions.
sql/opt_range.cc:
  Fixed get_quick_select_for_ref() function to not use bzero'ed MEM_ROOT for allocation.
  Also QUICK_RANGEs created in this function should be created in memory root of QUICK_SELECT.
sql/sql_class.cc:
  Implementation of Monty's idea about clear_alloc_root() optimization and cleanup of work 
  with memory roots in THD/Statement/Item_arena.
  Now we are always initing THD::transaction.mem_root and THD::mem_root in THD constructor 
  (without memory allocation and with minimal block size) and then later change their
  parameters in THD::init_for_queries() (this is partially because we can't allocate anything
  on bzero'ed memory roots anymore).
  Item_arena() constructor is now trivial and is used only then Item_arena is created as backup
  storage for other Item_arena (we use Item_arena(bool) now if it is part of Statement).
  Both trivial Item_arena constructor and destructor are now inline.
  Removed unneeded clear_alloc_root from Item_arena::restore_backup_item_arena().
sql/sql_class.h:
  Both trivial Item_arena constructor and destructor are now inline.
  Commented various Item_arena constructors.

1 files changed, 6 insertions, 2 deletions

diff --git a/sql/opt_range.cc b/sql/opt_range.cc
index 27e8e9c11e7..9c5b0235767 100644
--- a/sql/opt_range.cc
+++ b/sql/opt_range.cc
@@ -2554,7 +2554,8 @@ static bool null_part_in_key(KEY_PART *key_part, const char *key, uint length)
 
 QUICK_SELECT *get_quick_select_for_ref(THD *thd, TABLE *table, TABLE_REF *ref)
 {
-  QUICK_SELECT *quick=new QUICK_SELECT(thd, table, ref->key, 1);
+  MEM_ROOT *old_root= my_pthread_getspecific_ptr(MEM_ROOT*, THR_MALLOC);
+  QUICK_SELECT *quick= new QUICK_SELECT(thd, table, ref->key);
   KEY *key_info = &table->key_info[ref->key];
   KEY_PART *key_part;
   QUICK_RANGE *range;
@@ -2566,7 +2567,7 @@ QUICK_SELECT *get_quick_select_for_ref(THD *thd, TABLE *table, TABLE_REF *ref)
   {
     if (thd->is_fatal_error)
       goto err;					// out of memory
-    return quick;				// empty range
+    goto ok;                                    // empty range
   }
 
   if (!(range= new QUICK_RANGE()))
@@ -2613,9 +2614,12 @@ QUICK_SELECT *get_quick_select_for_ref(THD *thd, TABLE *table, TABLE_REF *ref)
       goto err;
   }
 
+ok:
+  my_pthread_setspecific_ptr(THR_MALLOC, old_root);
   return quick;
 
 err:
+  my_pthread_setspecific_ptr(THR_MALLOC, old_root);
   delete quick;
   return 0;
 }