Bug#27759: Wrong DATE/DATETIME comparison in LEAST()/GREATEST() functions.

The LEAST/GREATEST functions compared DATE/DATETIME values as
strings which in some cases could lead to a wrong result.

A new member function called cmp_datetimes() is added to the
Item_func_min_max class. It compares arguments in DATETIME context
and returns index of the least/greatest argument.
The Item_func_min_max::fix_length_and_dec() function now detects when
arguments should be compared in DATETIME context and sets the newly
added flag compare_as_dates. It indicates that the cmp_datetimes() function
should be called to get a correct result.
Item_func_min_max::val_xxx() methods are corrected to call the
cmp_datetimes() function when needed.
Objects of the Item_splocal class now stores and reports correct original
field type.

1 files changed, 9 insertions, 1 deletions

diff --git a/sql/item_func.h b/sql/item_func.h
index ec5d6bcda02..99e5328c39c 100644
--- a/sql/item_func.h
+++ b/sql/item_func.h
@@ -693,15 +693,23 @@ class Item_func_min_max :public Item_func
   Item_result cmp_type;
   String tmp_value;
   int cmp_sign;
+  /* TRUE <=> arguments should be compared in the DATETIME context. */
+  bool compare_as_dates;
+  /* An item used for issuing warnings while string to DATETIME conversion. */
+  Item *datetime_item;
+  THD *thd;
+
 public:
   Item_func_min_max(List<Item> &list,int cmp_sign_arg) :Item_func(list),
-    cmp_type(INT_RESULT), cmp_sign(cmp_sign_arg) {}
+    cmp_type(INT_RESULT), cmp_sign(cmp_sign_arg), compare_as_dates(FALSE),
+    datetime_item(0) {}
   double val_real();
   longlong val_int();
   String *val_str(String *);
   my_decimal *val_decimal(my_decimal *);
   void fix_length_and_dec();
   enum Item_result result_type () const { return cmp_type; }
+  uint cmp_datetimes(ulonglong *value);
 };
 
 class Item_func_min :public Item_func_min_max