MDEV-8918 Wrong result for CAST(AVG(bigint_column) AS SIGNED)

- Moving Item_xxx_field declarations after Item_sum_xxx declarations,
  so Item_xxx_field constructors can be defined directly in item_sum.h
  rather than item_sum.cc. This removes some duplicate code, e.g.
  initialization of the following members at constructor time:
  name, decimals, max_length, unsigned_flag, field, maybe_null.
- Adding Item_sum_field as a common parent for Item_avg_field and
  Item_variance_field
- Deriving Item_sum_field directly from Item rather that Item_result_field,
  as Item_sum_field descendants do not need anything from Item_result_field.
- Removing hybrid infrastructure from Item_avg_field,
  adding Item_avg_field_decimal and Item_avg_field_double instead,
  as desired result type is already known at constructor time
  (not only at fix_fields time). This simplifies the code.
- Changing Item_avg_field_decimal::val_int() to call val_int_from_decimal()
  instead of doing { return (longlong) rint(val_real()); }
  This is the fix itself.

mysql-test/r/func_group.result

@@ -2283,3 +2283,20 @@ Warnings:
 Warning	1292	Truncated incorrect INTEGER value: 'x'
 Warning	1292	Truncated incorrect DOUBLE value: 'x'
 Warning	1292	Truncated incorrect DECIMAL value: 'x'
+#
+#  MDEV-8918 Wrong result for CAST(AVG(a) AS SIGNED)
+#
+CREATE TABLE t1 (id INT, a BIGINT);
+INSERT INTO t1 VALUES (1,0x7FFFFFFFFFFFFFFF),(2,0x7FFFFFFFFFFFFFFF);
+SELECT id, AVG(a) AS avg, CAST(MIN(a) AS SIGNED) AS cast_min FROM t1 GROUP BY id HAVING avg!=123 ORDER BY id;
+id	avg	cast_min
+1	9223372036854775807.0000	9223372036854775807
+2	9223372036854775807.0000	9223372036854775807
+SELECT id, AVG(a) AS avg, CAST(AVG(a) AS SIGNED) AS cast_avg FROM t1 GROUP BY id HAVING avg!=123 ORDER BY id;
+id	avg	cast_avg
+1	9223372036854775807.0000	9223372036854775807
+2	9223372036854775807.0000	9223372036854775807
+DROP TABLE t1;
+#
+# End of 10.1 tests
+#

mysql-test/t/func_group.test

@@ -1570,3 +1570,17 @@ DROP TABLE t1,t2,t3,t4,t5,t6;
 --echo # MDEV-8852 Implicit or explicit CAST from MAX(string) to INT,DOUBLE,DECIMAL does not produce warnings
 --echo #
 SELECT MAX('x') << 1, CAST(MAX('x') AS DOUBLE), CAST(MAX('x') AS DECIMAL);
+
+
+--echo #
+--echo #  MDEV-8918 Wrong result for CAST(AVG(a) AS SIGNED)
+--echo #
+CREATE TABLE t1 (id INT, a BIGINT);
+INSERT INTO t1 VALUES (1,0x7FFFFFFFFFFFFFFF),(2,0x7FFFFFFFFFFFFFFF);
+SELECT id, AVG(a) AS avg, CAST(MIN(a) AS SIGNED) AS cast_min FROM t1 GROUP BY id HAVING avg!=123 ORDER BY id;
+SELECT id, AVG(a) AS avg, CAST(AVG(a) AS SIGNED) AS cast_avg FROM t1 GROUP BY id HAVING avg!=123 ORDER BY id;
+DROP TABLE t1;
+
+--echo #
+--echo # End of 10.1 tests
+--echo #

sql/item_sum.cc

@@ -2502,7 +2502,10 @@ void Item_sum_avg::update_field()
 
 Item *Item_sum_avg::result_item(THD *thd, Field *field)
 {
-  return new (thd->mem_root) Item_avg_field(thd, hybrid_type, this);
+  return
+    hybrid_type == DECIMAL_RESULT ?
+    (Item_avg_field*) new (thd->mem_root) Item_avg_field_decimal(thd, this) :
+    (Item_avg_field*) new (thd->mem_root) Item_avg_field_double(thd, this);
 }
 
 
@@ -2620,36 +2623,13 @@ Item_sum_hybrid::min_max_update_decimal_field()
 }
 
 
-Item_avg_field::Item_avg_field(THD *thd, Item_result res_type,
-                               Item_sum_avg *item):
-  Item_result_field(thd)
-{
-  name=item->name;
-  decimals=item->decimals;
-  max_length= item->max_length;
-  unsigned_flag= item->unsigned_flag;
-  field=item->result_field;
-  maybe_null=1;
-  hybrid_type= res_type;
-  prec_increment= item->prec_increment;
-  if (hybrid_type == DECIMAL_RESULT)
-  {
-    f_scale= item->f_scale;
-    f_precision= item->f_precision;
-    dec_bin_size= item->dec_bin_size;
-  }
-}
-
-double Item_avg_field::val_real()
+double Item_avg_field_double::val_real()
 {
   // fix_fields() never calls for this Item
   double nr;
   longlong count;
   uchar *res;
 
-  if (hybrid_type == DECIMAL_RESULT)
-    return val_real_from_decimal();
-
   float8get(nr,field->ptr);
   res= (field->ptr+sizeof(double));
   count= sint8korr(res);
@@ -2660,18 +2640,9 @@ double Item_avg_field::val_real()
 }
 
 
-longlong Item_avg_field::val_int()
-{
-  return (longlong) rint(val_real());
-}
-
-
-my_decimal *Item_avg_field::val_decimal(my_decimal *dec_buf)
+my_decimal *Item_avg_field_decimal::val_decimal(my_decimal *dec_buf)
 {
   // fix_fields() never calls for this Item
-  if (hybrid_type == REAL_RESULT)
-    return val_decimal_from_real(dec_buf);
-
   longlong count= sint8korr(field->ptr + dec_bin_size);
   if ((null_value= !count))
     return 0;
@@ -2686,21 +2657,6 @@ my_decimal *Item_avg_field::val_decimal(my_decimal *dec_buf)
 }
 
 
-String *Item_avg_field::val_str(String *str)
-{
-  // fix_fields() never calls for this Item
-  if (hybrid_type == DECIMAL_RESULT)
-    return val_string_from_decimal(str);
-  return val_string_from_real(str);
-}
-
-
-Item_std_field::Item_std_field(THD *thd, Item_sum_std *item):
-  Item_variance_field(thd, item)
-{
-}
-
-
 double Item_std_field::val_real()
 {
   double nr;
@@ -2711,19 +2667,6 @@ double Item_std_field::val_real()
 }
 
 
-Item_variance_field::Item_variance_field(THD *thd, Item_sum_variance *item):
-  Item_result_field(thd)
-{
-  name=item->name;
-  decimals=item->decimals;
-  max_length=item->max_length;
-  unsigned_flag= item->unsigned_flag;
-  field=item->result_field;
-  maybe_null=1;
-  sample= item->sample;
-}
-
-
 double Item_variance_field::val_real()
 {
   // fix_fields() never calls for this Item

sql/item_sum.h

@@ -820,37 +820,6 @@ class Item_sum_count :public Item_sum_int
 };
 
 
-/* Item to get the value of a stored sum function */
-
-class Item_sum_avg;
-
-class Item_avg_field :public Item_result_field
-{
-  Field *field;
-  Item_result hybrid_type;
-  uint f_precision, f_scale, dec_bin_size;
-  uint prec_increment;
-public:
-  Item_avg_field(THD *thd, Item_result res_type, Item_sum_avg *item);
-  enum Type type() const { return FIELD_AVG_ITEM; }
-  double val_real();
-  longlong val_int();
-  my_decimal *val_decimal(my_decimal *);
-  bool is_null() { update_null_value(); return null_value; }
-  String *val_str(String*);
-  enum_field_types field_type() const
-  {
-    return hybrid_type == DECIMAL_RESULT ?
-      MYSQL_TYPE_NEWDECIMAL : MYSQL_TYPE_DOUBLE;
-  }
-  enum Item_result result_type () const { return hybrid_type; }
-  bool check_vcol_func_processor(uchar *int_arg) 
-  {
-    return trace_unsupported_by_check_vcol_func_processor("avg_field");
-  }
-};
-
-
 class Item_sum_avg :public Item_sum_sum
 {
 public:
@@ -894,31 +863,6 @@ public:
   }
 };
 
-class Item_sum_variance;
-
-class Item_variance_field :public Item_result_field
-{
-  Field *field;
-  uint sample;
-public:
-  Item_variance_field(THD *thd, Item_sum_variance *item);
-  enum Type type() const {return FIELD_VARIANCE_ITEM; }
-  double val_real();
-  longlong val_int()
-  { /* can't be fix_fields()ed */ return (longlong) rint(val_real()); }
-  String *val_str(String *str)
-  { return val_string_from_real(str); }
-  my_decimal *val_decimal(my_decimal *dec_buf)
-  { return val_decimal_from_real(dec_buf); }
-  bool is_null() { update_null_value(); return null_value; }
-  enum_field_types field_type() const { return MYSQL_TYPE_DOUBLE; }
-  enum Item_result result_type () const { return REAL_RESULT; }
-  bool check_vcol_func_processor(uchar *int_arg) 
-  {
-    return trace_unsupported_by_check_vcol_func_processor("var_field");
-  }
-};
-
 
 /*
   variance(a) =
@@ -977,16 +921,6 @@ public:
   }
 };
 
-class Item_sum_std;
-
-class Item_std_field :public Item_variance_field
-{
-public:
-  Item_std_field(THD *thd, Item_sum_std *item);
-  enum Type type() const { return FIELD_STD_ITEM; }
-  double val_real();
-};
-
 /*
    standard_deviation(a) = sqrt(variance(a))
 */
@@ -1144,6 +1078,117 @@ class Item_sum_xor :public Item_sum_bit
 };
 
 
+/* Items to get the value of a stored sum function */
+
+class Item_sum_field :public Item
+{
+protected:
+  Field *field;
+public:
+  Item_sum_field(THD *thd, Item_sum *item)
+    :Item(thd), field(item->result_field)
+  {
+    name= item->name;
+    maybe_null= true;
+    decimals= item->decimals;
+    max_length= item->max_length;
+    unsigned_flag= item->unsigned_flag;
+  }
+  table_map used_tables() const { return (table_map) 1L; }
+  Field *get_tmp_table_field() { DBUG_ASSERT(0); return NULL; }
+  Field *tmp_table_field(TABLE *) { DBUG_ASSERT(0); return NULL; }
+  void set_result_field(Field *) { DBUG_ASSERT(0); }
+  void save_in_result_field(bool no_conversions) { DBUG_ASSERT(0); }
+};
+
+
+class Item_avg_field :public Item_sum_field
+{
+protected:
+  uint prec_increment;
+public:
+  Item_avg_field(THD *thd, Item_sum_avg *item)
+   :Item_sum_field(thd, item), prec_increment(item->prec_increment)
+  { }
+  enum Type type() const { return FIELD_AVG_ITEM; }
+  bool is_null() { update_null_value(); return null_value; }
+  bool check_vcol_func_processor(uchar *int_arg)
+  {
+    return trace_unsupported_by_check_vcol_func_processor("avg_field");
+  }
+};
+
+
+class Item_avg_field_double :public Item_avg_field
+{
+public:
+  Item_avg_field_double(THD *thd, Item_sum_avg *item)
+   :Item_avg_field(thd, item)
+  { }
+  enum_field_types field_type() const { return MYSQL_TYPE_DOUBLE; }
+  enum Item_result result_type () const { return REAL_RESULT; }
+  longlong val_int() { return (longlong) rint(val_real()); }
+  my_decimal *val_decimal(my_decimal *dec) { return val_decimal_from_real(dec); }
+  String *val_str(String *str) { return val_string_from_real(str); }
+  double val_real();
+};
+
+
+class Item_avg_field_decimal :public Item_avg_field
+{
+  uint f_precision, f_scale, dec_bin_size;
+public:
+  Item_avg_field_decimal(THD *thd, Item_sum_avg *item)
+   :Item_avg_field(thd, item),
+    f_precision(item->f_precision),
+    f_scale(item->f_scale),
+    dec_bin_size(item->dec_bin_size)
+  { }
+  enum_field_types field_type() const { return MYSQL_TYPE_NEWDECIMAL; }
+  enum Item_result result_type () const { return DECIMAL_RESULT; }
+  double val_real() { return val_real_from_decimal(); }
+  longlong val_int() { return val_int_from_decimal(); }
+  String *val_str(String *str) { return val_string_from_decimal(str); }
+  my_decimal *val_decimal(my_decimal *);
+};
+
+
+class Item_variance_field :public Item_sum_field
+{
+  uint sample;
+public:
+  Item_variance_field(THD *thd, Item_sum_variance *item)
+   :Item_sum_field(thd, item), sample(item->sample)
+  { }
+  enum Type type() const {return FIELD_VARIANCE_ITEM; }
+  double val_real();
+  longlong val_int()
+  { /* can't be fix_fields()ed */ return (longlong) rint(val_real()); }
+  String *val_str(String *str)
+  { return val_string_from_real(str); }
+  my_decimal *val_decimal(my_decimal *dec_buf)
+  { return val_decimal_from_real(dec_buf); }
+  bool is_null() { update_null_value(); return null_value; }
+  enum_field_types field_type() const { return MYSQL_TYPE_DOUBLE; }
+  enum Item_result result_type () const { return REAL_RESULT; }
+  bool check_vcol_func_processor(uchar *int_arg)
+  {
+    return trace_unsupported_by_check_vcol_func_processor("var_field");
+  }
+};
+
+
+class Item_std_field :public Item_variance_field
+{
+public:
+  Item_std_field(THD *thd, Item_sum_std *item)
+   :Item_variance_field(thd, item)
+  { }
+  enum Type type() const { return FIELD_STD_ITEM; }
+  double val_real();
+};
+
+
 /*
   User defined aggregates
 */