1 files changed, 24 insertions, 19 deletions

diff --git a/sql/item_sum.h b/sql/item_sum.h
index 27748d214dd..f074cc6c822 100644
--- a/sql/item_sum.h
+++ b/sql/item_sum.h
@@ -57,19 +57,8 @@ protected:
   /* the aggregate function class to act on */
   Item_sum *item_sum;
 
-  /**
-    When feeding back the data in endup() from Unique/temp table back to
-    Item_sum::add() methods we must read the data from Unique (and not 
-    recalculate the functions that are given as arguments to the aggregate
-    function. 
-    This flag is to tell the add() methods to take the data from the Unique
-    instead by calling the relevant val_..() method
-  */
-
-  bool use_distinct_values;
-
 public:
-  Aggregator (Item_sum *arg): item_sum(arg), use_distinct_values(FALSE) {}
+  Aggregator (Item_sum *arg): item_sum(arg) {}
   virtual ~Aggregator () {}                   /* Keep gcc happy */
 
   enum Aggregator_type { SIMPLE_AGGREGATOR, DISTINCT_AGGREGATOR }; 
@@ -106,10 +95,16 @@ public:
   /** Floating point value of being-aggregated argument */
   virtual double arg_val_real() = 0;
   /**
-     NULLness of being-aggregated argument; can be called only after
-     arg_val_decimal() or arg_val_real().
+    NULLness of being-aggregated argument.
+
+    @param use_null_value Optimization: to determine if the argument is NULL
+    we must, in the general case, call is_null() on it, which itself might
+    call val_*() on it, which might be costly. If you just have called
+    arg_val*(), you can pass use_null_value=true; this way, arg_is_null()
+    might avoid is_null() and instead do a cheap read of the Item's null_value
+    (updated by arg_val*()).
   */
-  virtual bool arg_is_null() = 0;
+  virtual bool arg_is_null(bool use_null_value) = 0;
 };
 
 
@@ -495,7 +490,7 @@ public:
 
   Item *get_arg(uint i) { return args[i]; }
   Item *set_arg(uint i, THD *thd, Item *new_val);
-  uint get_arg_count() { return arg_count; }
+  uint get_arg_count() const { return arg_count; }
 
   /* Initialization of distinct related members */
   void init_aggregator()
@@ -626,10 +621,20 @@ class Aggregator_distinct : public Aggregator
   */
   bool always_null;
 
+  /**
+    When feeding back the data in endup() from Unique/temp table back to
+    Item_sum::add() methods we must read the data from Unique (and not
+    recalculate the functions that are given as arguments to the aggregate
+    function.
+    This flag is to tell the arg_*() methods to take the data from the Unique
+    instead of calling the relevant val_..() method.
+  */
+  bool use_distinct_values;
+
 public:
   Aggregator_distinct (Item_sum *sum) :
     Aggregator(sum), table(NULL), tmp_table_param(NULL), tree(NULL),
-    always_null(FALSE) {}
+    always_null(false), use_distinct_values(false) {}
   virtual ~Aggregator_distinct ();
   Aggregator_type Aggrtype() { return DISTINCT_AGGREGATOR; }
 
@@ -639,7 +644,7 @@ public:
   void endup();
   virtual my_decimal *arg_val_decimal(my_decimal * value);
   virtual double arg_val_real();
-  virtual bool arg_is_null();
+  virtual bool arg_is_null(bool use_null_value);
 
   bool unique_walk_function(void *element);
   bool unique_walk_function_for_count(void *element);
@@ -666,7 +671,7 @@ public:
   void endup() {};
   virtual my_decimal *arg_val_decimal(my_decimal * value);
   virtual double arg_val_real();
-  virtual bool arg_is_null();
+  virtual bool arg_is_null(bool use_null_value);
 };