MDEV-13240 Wrong warning with MAX(datetime_field) OVER (...)

The problem resided in Item_window_func implementation,
and it was revealed by bb-10.2-ext specific changes:

Item_window_func::save_in_field() works differently in bb-10.2-ext vs 10.2:

- 10.2 goes through val_str()
- bb-10.2-ext goes through get_date(), due to Type_handler related changes.
  get_date() tries to convert empty string to DATETIME, hence the warning.

During a discussion with Vicentiu, it was decided to fix
Item_window_func::val_xxx() to return NULL
(instead of an "empty" value, such as 0 for numbers and '' for strings)
when force_return_blank is set.

1 files changed, 17 insertions, 8 deletions

diff --git a/sql/item_windowfunc.h b/sql/item_windowfunc.h
index 77cbd556e60..9fe95ed6cee 100644
--- a/sql/item_windowfunc.h
+++ b/sql/item_windowfunc.h
@@ -839,13 +839,24 @@ public:
     read_value_from_result_field= true;
   }
 
+  bool is_null()
+  {
+    if (force_return_blank)
+      return true;
+
+    if (read_value_from_result_field)
+      return result_field->is_null();
+
+    return window_func()->is_null();
+  }
+
   double val_real() 
   {
     double res;
     if (force_return_blank)
     {
       res= 0.0;
-      null_value= false;
+      null_value= true;
     }
     else if (read_value_from_result_field)
     {
@@ -866,7 +877,7 @@ public:
     if (force_return_blank)
     {
       res= 0;
-      null_value= false;
+      null_value= true;
     }
     else if (read_value_from_result_field)
     {
@@ -886,9 +897,8 @@ public:
     String *res;
     if (force_return_blank)
     {
-      null_value= false;
-      str->length(0);
-      res= str;
+      null_value= true;
+      res= NULL;
     }
     else if (read_value_from_result_field)
     {
@@ -910,9 +920,8 @@ public:
     my_decimal *res;
     if (force_return_blank)
     {
-      my_decimal_set_zero(dec);
-      null_value= false;
-      res= dec;
+      null_value= true;
+      res= NULL;
     }
     else if (read_value_from_result_field)
     {