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:
  Added a test case for bug #43432.
mysql-test/t/union.test:
  Added a test case for bug #43432.
sql/field.cc:
  Replaced FLT_DIG+6 and DBL_DIG+7 with a symbolic constant.
sql/item.cc:
  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.
sql/mysql_priv.h:
  Added a symbolic constant for FLT_DIG+6 and DBL_DIG+7.

1 files changed, 15 insertions, 0 deletions

diff --git a/mysql-test/t/union.test b/mysql-test/t/union.test
index 44f21abda19..35b9be60f2b 100644
--- a/mysql-test/t/union.test
+++ b/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