Automerge.

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

@@ -1023,4 +1023,19 @@ 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

@@ -6969,18 +6969,26 @@ bool Item_type_holder::join_types(THD *thd, Item *item)
   {
     if (decimals != NOT_FIXED_DEC)
     {
-      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;
-        decimals= NOT_FIXED_DEC;
-      } 
-      if (fld_type == MYSQL_TYPE_DOUBLE && max_length > DBL_DIG + 2) 
+      /*
+        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)
       {
-        max_length= DBL_DIG + 7;
-        decimals= NOT_FIXED_DEC;
+        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= MAX_FLOAT_STR_LENGTH;
+          decimals= NOT_FIXED_DEC;
+        } 
+        else if (fld_type == MYSQL_TYPE_DOUBLE && max_length > DBL_DIG + 2)
+        {
+          max_length= MAX_DOUBLE_STR_LENGTH;
+          decimals= NOT_FIXED_DEC;
+        }
       }
     }
     else

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