Merge 10.5 into 10.6

sql/sql_select.h

@@ -178,6 +178,12 @@ typedef struct st_table_ref
   */
   bool          disable_cache;
 
+  /*
+    If true, this ref access was constructed from equalities generated by
+    LATERAL DERIVED (aka GROUP BY splitting) optimization
+  */
+  bool          uses_splitting;
+
   bool tmp_table_index_lookup_init(THD *thd, KEY *tmp_key, Item_iterator &it,
                                    bool value, uint skip= 0);
   bool is_access_triggered();