MDEV-9877: Window functions: wrong sort criteria is used

" The sort order for the sub-sequence of window functions starting
from the element marked by SORTORDER_CHANGE_FLAG up to the next
element marked by SORTORDER_CHANGE_FLAG must be taken from the
last element of the sub-sequence (not from the first one)."

mysql-test/r/win.result

@@ -1693,6 +1693,58 @@ EXPLAIN
     }
   }
 }
+explain format=json
+select 
+rank() over (partition by c order by a),
+count(*) over (partition by c)
+from t1;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "window_functions_computation": {
+      "sorts": {
+        "filesort": {
+          "sort_key": "t1.c, t1.a"
+        }
+      },
+      "temporary_table": {
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "rows": 3,
+          "filtered": 100
+        }
+      }
+    }
+  }
+}
+explain format=json
+select 
+count(*) over (partition by c),
+rank() over (partition by c order by a)
+from t1;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "window_functions_computation": {
+      "sorts": {
+        "filesort": {
+          "sort_key": "t1.c, t1.a"
+        }
+      },
+      "temporary_table": {
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "rows": 3,
+          "filtered": 100
+        }
+      }
+    }
+  }
+}
 drop table t1;
 #
 # MDEV-9847: Window functions: crash with big_tables=1

mysql-test/t/win.test

@@ -1053,13 +1053,17 @@ select
   row_number() over (order by a)
 from t1;
 
---disable_parsing
 explain format=json
 select 
   rank() over (partition by c order by a),
   count(*) over (partition by c)
 from t1;
---enable_parsing
+
+explain format=json
+select 
+  count(*) over (partition by c),
+  rank() over (partition by c order by a)
+from t1;
 
 drop table t1;
 

sql/sql_window.cc

@@ -1829,16 +1829,9 @@ bool Window_funcs_sort::setup(THD *thd, SQL_SELECT *sel,
                              List_iterator<Item_window_func> &it)
 {
   Item_window_func *win_func= it.peek();
-  Window_spec *spec = win_func->window_spec;
+  Item_window_func *prev_win_func;
 
-  ORDER* sort_order= concat_order_lists(thd->mem_root, 
-                                        spec->partition_list->first,
-                                        spec->order_list->first);
-  filesort= new (thd->mem_root) Filesort(sort_order, HA_POS_ERROR, true, NULL);
-  /* Apply the same condition that the subsequent sort has. */
-  filesort->select= sel;
-
-  do 
+  do
   {
     Window_func_runner *runner;
     if (!(runner= new Window_func_runner(win_func)) ||
@@ -1848,7 +1841,22 @@ bool Window_funcs_sort::setup(THD *thd, SQL_SELECT *sel,
     }
     runners.push_back(runner);
     it++;
+    prev_win_func= win_func;
   } while ((win_func= it.peek()) && !(win_func->marker & SORTORDER_CHANGE_FLAG));
+  
+  /*
+    The sort criteria must be taken from the last win_func in the group of
+    adjacent win_funcs that do not have SORTORDER_CHANGE_FLAG.
+  */
+  Window_spec *spec = prev_win_func->window_spec;
+
+  ORDER* sort_order= concat_order_lists(thd->mem_root, 
+                                        spec->partition_list->first,
+                                        spec->order_list->first);
+  filesort= new (thd->mem_root) Filesort(sort_order, HA_POS_ERROR, true, NULL);
+
+  /* Apply the same condition that the subsequent sort has. */
+  filesort->select= sel;
 
   return false;
 }