MDEV-23032 FLOOR()/CEIL() incorrectly calculate the precision of a DECIMAL(M,D) column

The code in Item_func_int_val::fix_length_and_dec_int_or_decimal()
calculated badly the result data type for FLOOR()/CEIL(), so for example
the decimal(38,10) input created a decimal(28,0) result.
That was not correct, because one extra integer digit is needed.
   floor(-9.9) -> -10
   ceil(9.9)   ->  10

Rewritting the code in a more straightforward way.
Additional changes:
- FLOOR() now takes into account the presence of the UNSIGNED
flag of the argument: FLOOR(unsigned decimal) does not need an extra digits.
- FLOOR()/CEILING() now preserve the unsigned flag in the result
  data type is decimal.
These changes give nicer data types.

1 files changed, 3 insertions, 0 deletions

diff --git a/sql/item_func.h b/sql/item_func.h
index 24601983f86..d1292c3e07a 100644
--- a/sql/item_func.h
+++ b/sql/item_func.h
@@ -1695,6 +1695,7 @@ public:
   Item_func_int_val(THD *thd, Item *a): Item_func_hybrid_field_type(thd, a) {}
   bool check_partition_func_processor(void *int_arg) { return FALSE; }
   bool check_vcol_func_processor(void *arg) { return FALSE; }
+  virtual decimal_round_mode round_mode() const= 0;
   void fix_length_and_dec_double();
   void fix_length_and_dec_int_or_decimal();
   void fix_length_and_dec_time()
@@ -1723,6 +1724,7 @@ class Item_func_ceiling :public Item_func_int_val
 public:
   Item_func_ceiling(THD *thd, Item *a): Item_func_int_val(thd, a) {}
   const char *func_name() const { return "ceiling"; }
+  decimal_round_mode round_mode() const { return CEILING; }
   longlong int_op();
   double real_op();
   my_decimal *decimal_op(my_decimal *);
@@ -1738,6 +1740,7 @@ class Item_func_floor :public Item_func_int_val
 public:
   Item_func_floor(THD *thd, Item *a): Item_func_int_val(thd, a) {}
   const char *func_name() const { return "floor"; }
+  decimal_round_mode round_mode() const { return FLOOR; }
   longlong int_op();
   double real_op();
   my_decimal *decimal_op(my_decimal *);