MDEV-23366 ROUND(18446744073709551615,rand()*0) returns a wrong result

Changing that in case of *INT and hex hybrid input:
- ROUND(x,NULL) creates a column with the same type as x.
  The old code created a DOUBLE column, which was not relevant at all.
  This change simplifies the code a lot.

- ROUND(x,non_constant) creates a column of the INT, BIGINT or DECIMAL
  data type (depending on the exact type of x).
  The old code created a column of the DOUBLE data type,
  which lead to precision loss. Hence MDEV-23366.

- ROUND(bigint_30,negative_constant) creates a column of the DECIMAL(30,0)
  data type. The old code created DECIMAL(29,0), which looked strange:
  the data type promoted to a higher one, but max length reduced.
  Now the length attribute is preserved.

1 files changed, 2 insertions, 0 deletions

diff --git a/sql/item_func.h b/sql/item_func.h
index ea00a0cb033..24601983f86 100644
--- a/sql/item_func.h
+++ b/sql/item_func.h
@@ -1754,6 +1754,7 @@ class Item_func_round :public Item_func_hybrid_field_type
   bool truncate;
   void fix_length_and_dec_decimal(uint decimals_to_set);
   void fix_length_and_dec_double(uint decimals_to_set);
+  bool test_if_length_can_increase();
 public:
   Item_func_round(THD *thd, Item *a, Item *b, bool trunc_arg)
     :Item_func_hybrid_field_type(thd, a, b), truncate(trunc_arg) {}
@@ -1777,6 +1778,7 @@ public:
   void fix_arg_int(const Type_handler *preferred,
                    const Type_std_attributes *preferred_attributes,
                    bool use_decimal_on_length_increase);
+  void fix_arg_hex_hybrid();
   void fix_arg_double();
   void fix_arg_time();
   void fix_arg_datetime();