Fix for Bug#57932 "query with avg returns incorrect results":

when there was one NULL value, AVG(DISTINCT) could forget about other values.
See commit comment of item_sum.cc.

mysql-test/r/func_group.result

@@ -1746,3 +1746,18 @@ MAX(c1)	MIN(c1)
 -00:00:01	-00:00:01
 DROP TABLE t1;
 # End of the bug#56120
+#
+# Bug#57932 "query with AVG(DISTINCT) returns NULL if last
+# aggregated value was NULL"
+#
+CREATE TABLE t1 (col_int_nokey int(11));
+INSERT INTO t1 VALUES (7),(8),(NULL);
+SELECT AVG(DISTINCT col_int_nokey) FROM t1;
+AVG(DISTINCT col_int_nokey)
+7.5000
+SELECT AVG(DISTINCT outr.col_int_nokey) FROM t1 AS outr LEFT JOIN t1 AS outr2 ON
+outr.col_int_nokey = outr2.col_int_nokey;
+AVG(DISTINCT outr.col_int_nokey)
+7.5000
+DROP TABLE t1;
+# End of the bug#57932

mysql-test/t/func_group.test

@@ -1118,3 +1118,14 @@ SELECT MAX(c1),MIN(c1) FROM t1;
 DROP TABLE t1;
 --echo # End of the bug#56120
 
+--echo #
+--echo # Bug#57932 "query with AVG(DISTINCT) returns NULL if last
+--echo # aggregated value was NULL"
+--echo #
+CREATE TABLE t1 (col_int_nokey int(11));
+INSERT INTO t1 VALUES (7),(8),(NULL);
+SELECT AVG(DISTINCT col_int_nokey) FROM t1;
+SELECT AVG(DISTINCT outr.col_int_nokey) FROM t1 AS outr LEFT JOIN t1 AS outr2 ON
+outr.col_int_nokey = outr2.col_int_nokey;
+DROP TABLE t1;
+--echo # End of the bug#57932

sql/item_sum.cc

@@ -1325,27 +1325,11 @@ void Item_sum_sum::fix_length_and_dec()
 bool Item_sum_sum::add()
 {
   DBUG_ENTER("Item_sum_sum::add");
-  bool arg_is_null;
   if (hybrid_type == DECIMAL_RESULT)
   {
-    my_decimal value, *val;
-    if (aggr->use_distinct_values)
-    {
-      /* 
-        We are aggregating distinct rows. Get the value from the distinct 
-        table pointer
-      */
-      Aggregator_distinct *daggr= (Aggregator_distinct *)aggr;
-      val= daggr->table->field[0]->val_decimal (&value);
-      arg_is_null= daggr->table->field[0]->is_null();
-    }
-    else
-    {
-      /* non-distinct aggregation */
-      val= args[0]->val_decimal(&value);
-      arg_is_null= args[0]->null_value;
-    }
-    if (!arg_is_null)
+    my_decimal value;
+    const my_decimal *val= aggr->arg_val_decimal(&value);
+    if (!aggr->arg_is_null())
     {
       my_decimal_add(E_DEC_FATAL_ERROR, dec_buffs + (curr_dec_buff^1),
                      val, dec_buffs + curr_dec_buff);
@@ -1355,25 +1339,8 @@ bool Item_sum_sum::add()
   }
   else
   {
-    double val;
-    if (aggr->use_distinct_values)
-    {
-      /* 
-        We are aggregating distinct rows. Get the value from the distinct 
-        table pointer
-      */
-      Aggregator_distinct *daggr= (Aggregator_distinct *)aggr;
-      val= daggr->table->field[0]->val_real ();
-      arg_is_null= daggr->table->field[0]->is_null();
-    }
-    else
-    {
-      /* non-distinct aggregation */
-      val= args[0]->val_real();
-      arg_is_null= args[0]->null_value;
-    }
-    sum+= val;
-    if (!arg_is_null)
+    sum+= aggr->arg_val_real();
+    if (!aggr->arg_is_null())
       null_value= 0;
   }
   DBUG_RETURN(0);
@@ -1471,6 +1438,45 @@ Aggregator_distinct::~Aggregator_distinct()
 }
 
 
+my_decimal *Aggregator_simple::arg_val_decimal(my_decimal *value)
+{
+  return item_sum->args[0]->val_decimal(value);
+}
+
+
+double Aggregator_simple::arg_val_real()
+{
+  return item_sum->args[0]->val_real();
+}
+
+
+bool Aggregator_simple::arg_is_null()
+{
+  return item_sum->args[0]->null_value;
+}
+
+
+my_decimal *Aggregator_distinct::arg_val_decimal(my_decimal * value)
+{
+  return use_distinct_values ? table->field[0]->val_decimal(value) :
+    item_sum->args[0]->val_decimal(value);
+}
+
+
+double Aggregator_distinct::arg_val_real()
+{
+  return use_distinct_values ? table->field[0]->val_real() :
+    item_sum->args[0]->val_real();
+}
+
+
+bool Aggregator_distinct::arg_is_null()
+{
+  return use_distinct_values ? table->field[0]->is_null() :
+    item_sum->args[0]->null_value;
+}
+
+
 Item *Item_sum_count::copy_or_same(THD* thd)
 {
   return new (thd->mem_root) Item_sum_count(thd, this);
@@ -1576,7 +1582,7 @@ bool Item_sum_avg::add()
 {
   if (Item_sum_sum::add())
     return TRUE;
-  if (!args[0]->null_value)
+  if (!aggr->arg_is_null())
     count++;
   return FALSE;
 }

sql/item_sum.h

@@ -101,6 +101,15 @@ public:
   */
   virtual void endup() = 0;
 
+  /** Decimal value of being-aggregated argument */
+  virtual my_decimal *arg_val_decimal(my_decimal * value) = 0;
+  /** 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().
+  */
+  virtual bool arg_is_null() = 0;
 };
 
 
@@ -304,6 +313,7 @@ class st_select_lex;
 class Item_sum :public Item_result_field
 {
   friend class Aggregator_distinct;
+  friend class Aggregator_simple;
 
 protected:
   /**
@@ -600,6 +610,9 @@ public:
   void clear(); 
   bool add();
   void endup();
+  virtual my_decimal *arg_val_decimal(my_decimal * value);
+  virtual double arg_val_real();
+  virtual bool arg_is_null();
 
   bool unique_walk_function(void *element);
   static int composite_key_cmp(void* arg, uchar* key1, uchar* key2);
@@ -623,6 +636,9 @@ public:
   void clear() { item_sum->clear(); }
   bool add() { return item_sum->add(); }
   void endup() {};
+  virtual my_decimal *arg_val_decimal(my_decimal * value);
+  virtual double arg_val_real();
+  virtual bool arg_is_null();
 };