Fix for bug #43432: Union on floats does unnecessary rounding

  
UNION could convert fixed-point FLOAT(M,D)/DOUBLE(M,D) columns  
to FLOAT/DOUBLE when aggregating data types from the SELECT  
substatements. While there is nothing particularly wrong with  
this behavior, especially when M is greater than the hardware  
precision limits, it could be confusing in cases when all  
SELECT statements in a union have the same  
FLOAT(M,D)/DOUBLE(M,D) columns with equal precision  
specifications listed in the same position.  
  
Since the manual is quite vague on what data type should be  
returned in such cases, the bug was fixed by implementing the  
most 'expected' behavior: do not convert FLOAT(M,D)/DOUBLE(M,D)  
to anything else if all SELECT statements in a UNION have the  
same precision for that column.  

mysql-test/r/union.result

@@ -1506,4 +1506,16 @@ DESC t6;
 Field	Type	Null	Key	Default	Extra
 NULL	int(11)	YES		NULL	
 DROP TABLE t1, t2, t3, t4, t5, t6;
+CREATE TABLE t1 (f FLOAT(9,6));
+CREATE TABLE t2 AS SELECT f FROM t1 UNION SELECT f FROM t1;
+SHOW FIELDS FROM t2;
+Field	Type	Null	Key	Default	Extra
+f	float(9,6)	YES		NULL	
+DROP TABLE t1, t2;
+CREATE TABLE t1(d DOUBLE(9,6));
+CREATE TABLE t2 AS SELECT d FROM t1 UNION SELECT d FROM t1;
+SHOW FIELDS FROM t2;
+Field	Type	Null	Key	Default	Extra
+d	double(9,6)	YES		NULL	
+DROP TABLE t1, t2;
 End of 5.0 tests

mysql-test/t/union.test

@@ -1000,4 +1000,19 @@ SELECT * FROM (SELECT * FROM (SELECT NULL)a) b UNION SELECT a FROM t1;
 DESC t6;
 
 DROP TABLE t1, t2, t3, t4, t5, t6;
+
+#
+# Bug #43432: Union on floats does unnecessary rounding
+#
+
+CREATE TABLE t1 (f FLOAT(9,6));
+CREATE TABLE t2 AS SELECT f FROM t1 UNION SELECT f FROM t1;
+SHOW FIELDS FROM t2;
+DROP TABLE t1, t2;
+
+CREATE TABLE t1(d DOUBLE(9,6));
+CREATE TABLE t2 AS SELECT d FROM t1 UNION SELECT d FROM t1;
+SHOW FIELDS FROM t2;
+DROP TABLE t1, t2;
+
 --echo End of 5.0 tests

sql/field.cc

@@ -8587,16 +8587,16 @@ bool create_field::init(THD *thd, char *fld_name, enum_field_types fld_type,
       else if (tmp_length > PRECISION_FOR_FLOAT)
       {
         sql_type= FIELD_TYPE_DOUBLE;
-        length= DBL_DIG+7; /* -[digits].E+### */
+        length= MAX_DOUBLE_STR_LENGTH; 
       }
       else
-        length= FLT_DIG+6; /* -[digits].E+## */
+        length= MAX_FLOAT_STR_LENGTH; 
       decimals= NOT_FIXED_DEC;
       break;
     }
     if (!fld_length && !fld_decimals)
     {
-      length=  FLT_DIG+6;
+      length=  MAX_FLOAT_STR_LENGTH;
       decimals= NOT_FIXED_DEC;
     }
     if (length < decimals &&

sql/item.cc

@@ -6948,21 +6948,29 @@ bool Item_type_holder::join_types(THD *thd, Item *item)
   case REAL_RESULT:
   {
     if (decimals != NOT_FIXED_DEC)
+    {
+      /*
+        For FLOAT(M,D)/DOUBLE(M,D) do not change precision
+         if both fields have the same M and D
+      */
+      if (item->max_length != max_length_orig ||
+          item->decimals != decimals_orig)
       {
         int delta1= max_length_orig - decimals_orig;
         int delta2= item->max_length - item->decimals;
         max_length= max(delta1, delta2) + decimals;
         if (fld_type == MYSQL_TYPE_FLOAT && max_length > FLT_DIG + 2)
         {
-        max_length= FLT_DIG + 6;
+          max_length= MAX_FLOAT_STR_LENGTH;
           decimals= NOT_FIXED_DEC;
         } 
-      if (fld_type == MYSQL_TYPE_DOUBLE && max_length > DBL_DIG + 2) 
+        else if (fld_type == MYSQL_TYPE_DOUBLE && max_length > DBL_DIG + 2)
         {
-        max_length= DBL_DIG + 7;
+          max_length= MAX_DOUBLE_STR_LENGTH;
           decimals= NOT_FIXED_DEC;
         }
       }
+    }
     else
       max_length= (fld_type == MYSQL_TYPE_FLOAT) ? FLT_DIG+6 : DBL_DIG+7;
     break;

sql/mysql_priv.h

@@ -251,6 +251,11 @@ MY_LOCALE *my_locale_by_number(uint number);
 #define PRECISION_FOR_DOUBLE 53
 #define PRECISION_FOR_FLOAT  24
 
+/* -[digits].E+## */
+#define MAX_FLOAT_STR_LENGTH (FLT_DIG + 6)
+/* -[digits].E+### */
+#define MAX_DOUBLE_STR_LENGTH (DBL_DIG + 7)
+
 /*
   Default time to wait before aborting a new client connection
   that does not respond to "initial server greeting" timely