MDEV-22702: Assertion `!field->is_null()' failed in my_decimal::my_decimal

With implicit grouping with window functions, we need to make sure that all the
fields inside the window functions are nullable as any non-aggregated field can
produce a NULL value.

mysql-test/r/win.result

@@ -3866,5 +3866,19 @@ NULL
 DROP VIEW v1;
 DROP TABLE t1,t2;
 #
+# MDEV-22702: Assertion `!field->is_null()' failed in my_decimal::my_decimal
+#
+CREATE TABLE t1(a INT, b DECIMAL(10, 0) NOT NULL);
+SELECT a, bit_or(min(a)) OVER (ORDER BY b) FROM t1;
+a	bit_or(min(a)) OVER (ORDER BY b)
+NULL	0
+# No implicit grouping here
+SELECT a, bit_or(a) OVER (ORDER BY b) FROM t1;
+a	bit_or(a) OVER (ORDER BY b)
+SELECT a, sum(a), bit_or(a) OVER (ORDER BY b) FROM t1;
+a	sum(a)	bit_or(a) OVER (ORDER BY b)
+NULL	NULL	0
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/win.test

@@ -2522,6 +2522,17 @@ SELECT NTH_VALUE(i1, i1) OVER (PARTITION BY i1) FROM v1;
 DROP VIEW v1;
 DROP TABLE t1,t2;
 
+--echo #
+--echo # MDEV-22702: Assertion `!field->is_null()' failed in my_decimal::my_decimal
+--echo #
+
+CREATE TABLE t1(a INT, b DECIMAL(10, 0) NOT NULL);
+SELECT a, bit_or(min(a)) OVER (ORDER BY b) FROM t1;
+--echo # No implicit grouping here
+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 # End of 10.2 tests
 --echo #

sql/sql_lex.h

@@ -1191,6 +1191,8 @@ class st_select_lex: public st_select_lex_node
   bool add_window_func(Item_window_func *win_func);
 
   bool have_window_funcs() const { return (window_funcs.elements !=0); }
+  uint get_number_of_window_funcs() const
+  { return window_funcs.elements; }
 
   bool cond_pushdown_is_allowed() const
   { return !have_window_funcs() && !olap && !explicit_limit; }

sql/sql_select.cc

@@ -770,6 +770,31 @@ JOIN::prepare(TABLE_LIST *tables_init,
         break;
       }
     }
+
+    /*
+      If the query has a window function with an aggregate function,
+      then we have a mix of elements with and without grouping.
+      Window function can be present in the SELECT LIST and the ORDER BY clause
+      so a separate check is made here (above we just iterate over the
+      SELECT LIST).
+
+      Window function is inherited from Item_sum class which means
+      each window function is registered as a sum item,
+      so a check needs to be any of the sum items are aggregate functions
+      or not.
+
+      Example:
+
+        (1) SELECT a, sum(a), row_number() OVER (ORDER BY b)  :=> sum_items= 2
+        (2) SELECT a, sum(a) OVER (ORDER BY b)                :=> sum_items= 1
+
+      In example (1) we have an aggregate function and a window function
+      In example (2) we have a window function (SUM function is used as
+      a window function here)
+    */
+    if (select_lex->have_window_funcs() &&
+        select_lex->get_number_of_window_funcs() < select_lex->n_sum_items)
+      mixed_implicit_grouping= true;
   }
 
   table_count= select_lex->leaf_tables.elements;