Bug #24791: Union with AVG-groups generates wrong results

The problem in this bug is when we create temporary tables. When
temporary tables are created for unions, there is some 
inferrence being carried out regarding the type of the column.
Whenever this column type is inferred to be REAL (i.e. FLOAT or
DOUBLE), MySQL will always try to maintain exact precision, and
if that is not possible (there are hardware limits, since FLOAT
and DOUBLE are stored as approximate values) will switch to
using approximate values. The problem here is that at this point
the information about number of significant digits is not 
available. Furthermore, the number of significant digits should
be increased for the AVG function, however, this was not properly 
handled. There are 4 parts to the problem:

#1: DOUBLE and FLOAT fields don't display their proper display 
lengths in max_display_length(). This is hard-coded as 53 for 
DOUBLE and 24 for FLOAT. Now changed to instead return the 
field_length.

#2: Type holders for temporary tables do not preserve the 
max_length of the Item's from which they are created, and is 
instead reverted to the 53 and 24 from above. This causes 
*all* fields to get non-fixed significant digits.

#3: AVG function does not update max_length (display length)
when updating number of decimals.

#4: The function that switches to non-fixed number of 
significant digits should use DBL_DIG + 2 or FLT_DIG + 2 as 
cut-off values (Since fixed precision does not use the 'e' 
notation)

Of these points, #1 is the controversial one, but this 
change is preferred and has been cleared with Monty. The 
function causes quite a few unit tests to blow up and they had
to b changed, but each one is annotated and motivated. We 
frequently see the magical 53 and 24 give way to more relevant
numbers.

mysql-test/r/create.result

@@ -447,8 +447,8 @@ t2	CREATE TABLE `t2` (
   `ifnull(c,c)` mediumint(8) default NULL,
   `ifnull(d,d)` int(11) default NULL,
   `ifnull(e,e)` bigint(20) default NULL,
-  `ifnull(f,f)` float(24,2) default NULL,
-  `ifnull(g,g)` double(53,3) default NULL,
+  `ifnull(f,f)` float(3,2) default NULL,
+  `ifnull(g,g)` double(4,3) default NULL,
   `ifnull(h,h)` decimal(5,4) default NULL,
   `ifnull(i,i)` year(4) default NULL,
   `ifnull(j,j)` date default NULL,

mysql-test/r/temp_table.result

@@ -152,3 +152,24 @@ SELECT * FROM t1;
 i
 DROP TABLE t1;
 End of 4.1 tests.
+CREATE TABLE t1 ( c FLOAT( 20, 14 ) );
+INSERT INTO t1 VALUES( 12139 );
+CREATE TABLE t2 ( c FLOAT(30,18) );
+INSERT INTO t2 VALUES( 123456 );
+SELECT AVG( c ) FROM t1 UNION SELECT 1;
+AVG( c )
+12139
+1
+SELECT 1 UNION SELECT AVG( c ) FROM t1;
+1
+1
+12139
+SELECT 1 UNION SELECT * FROM t2 UNION SELECT 1;
+1
+1
+123456
+SELECT c/1 FROM t1 UNION SELECT 1;
+c/1
+12139
+1
+DROP TABLE t1, t2;

mysql-test/r/type_float.result

@@ -92,7 +92,7 @@ show create table t2;
 Table	Create Table
 t2	CREATE TABLE `t2` (
   `col1` double default NULL,
-  `col2` double(53,5) default NULL,
+  `col2` double(22,5) default NULL,
   `col3` double default NULL,
   `col4` double default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
@@ -232,12 +232,12 @@ insert into t2 values ("1.23456780");
 create table t3 select * from t2 union select * from t1;
 select * from t3;
 d
-1.234567800
-100000000.000000000
+1.2345678
+100000000
 show create table t3;
 Table	Create Table
 t3	CREATE TABLE `t3` (
-  `d` double(22,9) default NULL
+  `d` double default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1, t2, t3;
 create table t1 select  105213674794682365.00 + 0.0 x;

mysql-test/t/temp_table.test

@@ -163,3 +163,19 @@ DROP TABLE t1;
 
 
 --echo End of 4.1 tests.
+
+#
+# Bug #24791: Union with AVG-groups generates wrong results
+#
+CREATE TABLE t1 ( c FLOAT( 20, 14 ) );
+INSERT INTO t1 VALUES( 12139 );
+
+CREATE TABLE t2 ( c FLOAT(30,18) );
+INSERT INTO t2 VALUES( 123456 );
+
+SELECT AVG( c ) FROM t1 UNION SELECT 1;
+SELECT 1 UNION SELECT AVG( c ) FROM t1;
+SELECT 1 UNION SELECT * FROM t2 UNION SELECT 1;
+SELECT c/1 FROM t1 UNION SELECT 1;
+
+DROP TABLE t1, t2;

sql/field.h

@@ -432,6 +432,7 @@ public:
 
   int store_decimal(const my_decimal *);
   my_decimal *val_decimal(my_decimal *);
+  uint32 max_display_length() { return field_length; }
 };
 
 
@@ -461,7 +462,6 @@ public:
   void overflow(bool negative);
   bool zero_pack() const { return 0; }
   void sql_type(String &str) const;
-  uint32 max_display_length() { return field_length; }
 };
 
 
@@ -719,7 +719,6 @@ public:
   void sort_string(char *buff,uint length);
   uint32 pack_length() const { return sizeof(float); }
   void sql_type(String &str) const;
-  uint32 max_display_length() { return 24; }
 };
 
 
@@ -762,7 +761,6 @@ public:
   void sort_string(char *buff,uint length);
   uint32 pack_length() const { return sizeof(double); }
   void sql_type(String &str) const;
-  uint32 max_display_length() { return 53; }
   uint size_of() const { return sizeof(*this); }
 };
 

sql/item.cc

@@ -6348,8 +6348,6 @@ Item_type_holder::Item_type_holder(THD *thd, Item *item)
   :Item(thd, item), enum_set_typelib(0), fld_type(get_real_type(item))
 {
   DBUG_ASSERT(item->fixed);
-
-  max_length= display_length(item);
   maybe_null= item->maybe_null;
   collation.set(item->collation);
   get_full_info(item);
@@ -6521,11 +6519,17 @@ bool Item_type_holder::join_types(THD *thd, Item *item)
     {
       int delta1= max_length_orig - decimals_orig;
       int delta2= item->max_length - item->decimals;
-      if (fld_type == MYSQL_TYPE_DECIMAL)
-        max_length= max(delta1, delta2) + decimals;
-      else
-        max_length= min(max(delta1, delta2) + decimals,
-                        (fld_type == MYSQL_TYPE_FLOAT) ? FLT_DIG+6 : DBL_DIG+7);
+      max_length= max(delta1, delta2) + decimals;
+      if (fld_type == MYSQL_TYPE_FLOAT && max_length > FLT_DIG + 2) 
+      {
+        max_length= FLT_DIG + 6;
+        decimals= NOT_FIXED_DEC;
+      } 
+      if (fld_type == MYSQL_TYPE_DOUBLE && max_length > DBL_DIG + 2) 
+      {
+        max_length= DBL_DIG + 7;
+        decimals= NOT_FIXED_DEC;
+      }
     }
     else
       max_length= (fld_type == MYSQL_TYPE_FLOAT) ? FLT_DIG+6 : DBL_DIG+7;

sql/item_sum.cc

@@ -1096,8 +1096,10 @@ void Item_sum_avg::fix_length_and_dec()
     f_scale=  args[0]->decimals;
     dec_bin_size= my_decimal_get_binary_size(f_precision, f_scale);
   }
-  else
+  else {
     decimals= min(args[0]->decimals + prec_increment, NOT_FIXED_DEC);
+    max_length= args[0]->max_length + prec_increment;
+  }
 }