fixed bug in Item_func_div::val_int() that broke all functions that

do val_int() on their arguments before starting the computation. 
Similar fixes are need for +-* and probably several other but I want
to make sure Monty is fine with my fix approach before changing a lot
of code. 

Amazingly,
this bug is not as critical as you would expect since very few functions do val_int()
on their arguments ( from_unixtime(), sec_to_time()), and those not
very frequently perform a computation on their floating point arguments.
which is probably why no one has yet reported this bug. Another 
possibility is that the result is usually wrong by no more than 5%,
which makes it hard to catch it. I found it when trying to compute mile
splits for 30:47 10K - it told me 5:07, and I knew it was wrong because
5:00 mile gives you 31:08. However, if I had not run as many 10K races,
I would have easily believed that 30:47 10K is a 5:07 mile pace and 
would not have noticed the bug.


mysql-test/r/func_time.result:
  another test for sec_to_time that exposes a long outstanding bug
mysql-test/t/func_time.test:
  another test for sec_to_time that exposes a long outstanding bug
sql/item_func.cc:
  fixed bug in Item_func_div::val_int()

1 files changed, 8 insertions, 5 deletions

diff --git a/sql/item_func.cc b/sql/item_func.cc
index 10298ce67f2..1d1a72d35eb 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -309,11 +309,14 @@ double Item_func_div::val()
 
 longlong Item_func_div::val_int()
 {
-  longlong value=args[0]->val_int();
-  longlong val2=args[1]->val_int();
-  if ((null_value= val2 == 0 || args[0]->null_value || args[1]->null_value))
-    return 0;
-  return value/val2;
+  // the integer result of division of two arguments needs to be computed
+  // as a type-cast division of val(), not as diviion of val_int() of each
+  // argument. For example, val_int(41.5/3.4) = val_int(12.206) = 12, but
+  // if you do val_int(41.5)/val_int(3.4), as in the old code, we get 42/3=
+  // 14, which is wrong. This would break sec_to_time(a/b),
+  // from_unixtime(a/b), and
+  // all functions that do val_int() on their arguments
+  return (longlong)val();
 }
 
 void Item_func_div::fix_length_and_dec()