MDEV-12851: Case with window functions query crashes server

The "is null" function performs one operation which no other Item_func
does, which is to update used tables during fix_length_and_dec().

This however can not be performed before window functions have had a
chance to resolve their order by and partition by definitions, which
happens after the initial setup_fields call. Consequently, do not call
Item_func_isnull update_used_tables during fix_length_and_dec().

There was another issue detected once the crash was resolved.
Because window functions did not implement is_null() method, we would
end up returning bad results for "is null" and "is not null" functions.
Implemented is_null() method for Item_windowfunc.

1 files changed, 0 insertions, 5 deletions

diff --git a/sql/item_cmpfunc.h b/sql/item_cmpfunc.h
index c81a1621f8a..2f98c68bb5f 100644
--- a/sql/item_cmpfunc.h
+++ b/sql/item_cmpfunc.h
@@ -1770,11 +1770,6 @@ public:
   Item_func_isnull(THD *thd, Item *a): Item_func_null_predicate(thd, a) {}
   longlong val_int();
   enum Functype functype() const { return ISNULL_FUNC; }
-  void fix_length_and_dec()
-  {
-    Item_func_null_predicate::fix_length_and_dec();
-    update_used_tables();
-  }
   const char *func_name() const { return "isnull"; }
   void print(String *str, enum_query_type query_type);
   enum precedence precedence() const { return CMP_PRECEDENCE; }