MDEV-23438: Assertion `!field->is_null()' failed in my_decimal::my_decimal...

MDEV-23438: Assertion `!field->is_null()' failed in my_decimal::my_decimal fails in ONLY_FULL_GROUP_BY mode

The issue here is query with aggregate function and non-aggregate field
in the SELECT LIST with ONLY_FULL_GROUP_BY was not disallowed.
In ONLY_FULL_GROUP_BY mode non-aggregate fields are only allowed inside an
aggregate functions or the non-aggregate fields are part of the GROUP BY clause.

In the query for the failing assert the non-aggregate field was inside
a WINDOW function and the window function was treated as an aggregate function
and so no error was thrown.

The fix would be to make sure to mark that a non-aggregate field is used inside a
window function and not an aggregate function and throw an error then.

mysql-test/r/win.result

@@ -3880,5 +3880,18 @@ a	sum(a)	bit_or(a) OVER (ORDER BY b)
 NULL	NULL	0
 DROP TABLE t1;
 #
+# MDEV-23438: Assertion `!field->is_null()' failed in my_decimal::my_decimal fails
+# in ONLY_FULL_GROUP_BY mode
+#
+CREATE TABLE t1(a INT, b DECIMAL(10, 0) NOT NULL);
+SET @save_sql_mode= @@sql_mode;
+SET sql_mode='ONLY_FULL_GROUP_BY';
+SELECT a, sum(a), bit_or(a) OVER (ORDER BY b) FROM t1;
+ERROR 42000: Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP columns is illegal if there is no GROUP BY clause
+SELECT sum(a),  last_value(b) OVER () FROM t1;
+ERROR 42000: Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP columns is illegal if there is no GROUP BY clause
+SET sql_mode= @save_sql_mode;
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/win.test

@@ -2533,6 +2533,21 @@ SELECT a, bit_or(a) OVER (ORDER BY b) FROM t1;
 SELECT a, sum(a), bit_or(a) OVER (ORDER BY b) FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-23438: Assertion `!field->is_null()' failed in my_decimal::my_decimal fails
+--echo # in ONLY_FULL_GROUP_BY mode
+--echo #
+
+CREATE TABLE t1(a INT, b DECIMAL(10, 0) NOT NULL);
+SET @save_sql_mode= @@sql_mode;
+SET sql_mode='ONLY_FULL_GROUP_BY';
+--error ER_MIX_OF_GROUP_FUNC_AND_FIELDS
+SELECT a, sum(a), bit_or(a) OVER (ORDER BY b) FROM t1;
+--error ER_MIX_OF_GROUP_FUNC_AND_FIELDS
+SELECT sum(a),  last_value(b) OVER () FROM t1;
+SET sql_mode= @save_sql_mode;
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/item_sum.cc

@@ -84,7 +84,9 @@ bool Item_sum::init_sum_func_check(THD *thd)
   /* Set a reference to the nesting set function if there is  any */
   in_sum_func= thd->lex->in_sum_func;
   /* Save a pointer to object to be used in items for nested set functions */
+  if (!window_func_sum_expr_flag)
     thd->lex->in_sum_func= this;
+
   nest_level= thd->lex->current_select->nest_level;
   ref_by= 0;
   aggr_level= -1;

sql/item_windowfunc.cc

@@ -93,6 +93,12 @@ Item_window_func::fix_fields(THD *thd, Item **ref)
     my_error(ER_NO_ORDER_LIST_IN_WINDOW_SPEC, MYF(0), window_func()->func_name());
     return true;
   }
+
+  /*
+    Mark that Item_sum is used as a window function
+  */
+  window_func()->mark_as_window_func_sum_expr();
+
   /*
     TODO: why the last parameter is 'ref' in this call? What if window_func
     decides to substitute itself for something else and does *ref=.... ? 

sql/sql_yacc.yy

@@ -10497,9 +10497,6 @@ window_func:
           simple_window_func
         |
           sum_expr
-          {
-            ((Item_sum *) $1)->mark_as_window_func_sum_expr();
-          }
         ;
 
 simple_window_func: