MDEV-9712 Performance degradation of nested NULLIF

10.1 introduced a problem:
Execution time for various recursive stages
(walk, update_used_table, and propagate_equal_fields)
in NULLIF is O(recursion_level^2), because complexity is
doubled on every recursion level when we copy args[0] to args[2].

This change fixes to avoid unnecessary recursion in:
- Item_func_nullif::walk
- Item_func_nullif::update_used_tables
- Item_func_nullif::propagate_equal_fields
when possible.

sql/item_cmpfunc.cc

@@ -2507,6 +2507,23 @@ void Item_func_nullif::split_sum_func(THD *thd, Item **ref_pointer_array,
 }
 
 
+bool Item_func_nullif::walk(Item_processor processor,
+                            bool walk_subquery, uchar *arg)
+{
+  /*
+    No needs to iterate through args[2] when it's just a copy of args[0].
+    See MDEV-9712 Performance degradation of nested NULLIF
+  */
+  uint tmp_count= arg_count == 2 || args[0] == args[2] ? 2 : 3;
+  for (uint i= 0; i < tmp_count; i++)
+  {
+    if (args[i]->walk(processor, walk_subquery, arg))
+      return true;
+  }
+  return (this->*processor)(arg);
+}
+
+
 void Item_func_nullif::update_used_tables()
 {
   if (m_cache)
@@ -2517,7 +2534,14 @@ void Item_func_nullif::update_used_tables()
   }
   else
   {
-    Item_func::update_used_tables();
+    /*
+      MDEV-9712 Performance degradation of nested NULLIF
+      No needs to iterate through args[2] when it's just a copy of args[0].
+    */
+    DBUG_ASSERT(arg_count == 3);
+    used_tables_and_const_cache_init();
+    used_tables_and_const_cache_update_and_join(args[0] == args[2] ? 2 : 3,
+                                                args);
   }
 }
 

sql/item_cmpfunc.h

@@ -1026,6 +1026,7 @@ class Item_func_nullif :public Item_func_hybrid_field_type
   String *str_op(String *str);
   my_decimal *decimal_op(my_decimal *);
   void fix_length_and_dec();
+  bool walk(Item_processor processor, bool walk_subquery, uchar *arg);
   uint decimal_precision() const { return args[2]->decimal_precision(); }
   const char *func_name() const { return "nullif"; }
   void print(String *str, enum_query_type query_type);
@@ -1037,10 +1038,18 @@ class Item_func_nullif :public Item_func_hybrid_field_type
   Item* propagate_equal_fields(THD *thd, const Context &ctx, COND_EQUAL *cond)
   {
     Context cmpctx(ANY_SUBST, cmp.compare_type(), cmp.compare_collation());
+    const Item *old0= args[0];
     args[0]->propagate_equal_fields_and_change_item_tree(thd, cmpctx,
                                                          cond, &args[0]);
     args[1]->propagate_equal_fields_and_change_item_tree(thd, cmpctx,
                                                          cond, &args[1]);
+    /*
+      MDEV-9712 Performance degradation of nested NULLIF
+      ANY_SUBST is more relaxed than IDENTITY_SUBST.
+      If ANY_SUBST did not change args[0],
+      then we can skip propagation for args[2].
+    */
+    if (old0 != args[0])
       args[2]->propagate_equal_fields_and_change_item_tree(thd,
                                                            Context_identity(),
                                                            cond, &args[2]);