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.