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, 14 insertions, 2 deletions

diff --git a/sql/sql_class.h b/sql/sql_class.h
index 387bba43cad..89bf2dde12e 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -441,11 +441,23 @@ public:
     STATEMENT, PREPARED_STATEMENT, STORED_PROCEDURE
   };
 
+  /*
+    This constructor is used only when Item_arena is created as
+    backup storage for another instance of Item_arena.
+  */
+  Item_arena() {};
+  /*
+    Create arena for already constructed THD using its variables as
+    parameters for memory root initialization.
+  */
   Item_arena(THD *thd);
-  Item_arena();
+  /*
+    Create arena and optionally init memory root with minimal values.
+    Particularly used if Item_arena is part of Statement.
+  */
   Item_arena(bool init_mem_root);
   virtual Type type() const;
-  virtual ~Item_arena();
+  virtual ~Item_arena() {};
 
   inline bool is_stmt_prepare() const { return (int)state < (int)PREPARED; }
   inline bool is_first_stmt_execute() const { return state == PREPARED; }