MDEV-15837: Assertion `item1->type() == Item::FIELD_ITEM && item2->type() == Item::FIELD_ITEM'

            failed in compare_order_elements function

The issue here is the function compare_order_lists() is called for the order by list of the window functions
so that those window function that can be computed together are adjacent.
So in the function compare_order_list we iterate over all the elements in the order list of the two functions and
compare the items in their order by clause.
The function compare_order_elements() is called for each item in the
order by clause. This function assumes that all the items that are in the order by list would be of the type
Item::FIELD_ITEM.

The case we have is that we have constants in the order by clause. We should ignore the constant and only compare
items of the type Item::FIELD_ITEM in compare_order_elements()

1 files changed, 16 insertions, 0 deletions

diff --git a/sql/sql_window.cc b/sql/sql_window.cc
index 465c6ae032c..310cf5bfd91 100644
--- a/sql/sql_window.cc
+++ b/sql/sql_window.cc
@@ -342,6 +342,22 @@ int compare_order_lists(SQL_I_List<ORDER> *part_list1,
   for ( ; elem1 && elem2; elem1= elem1->next, elem2= elem2->next)
   {
     int cmp;
+    // remove all constants as we don't need them for comparision
+    while(elem1 && ((*elem1->item)->real_item())->const_item())
+    {
+      elem1= elem1->next;
+      continue;
+    }
+
+    while(elem2 && ((*elem2->item)->real_item())->const_item())
+    {
+      elem2= elem2->next;
+      continue;
+    }
+
+    if (!elem1 || !elem2)
+      break;
+
     if ((cmp= compare_order_elements(elem1, elem2)))
       return cmp;
   }