Hello Sergei, Please have a look into a draft fix for MDEV-9712. The problem happens because in 10.1 execution time for various recursive stages (like 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 fix simplifies some stages to make them faster. It reduced execution time from 144 seconds to 12 seconds. The idea is just not to iterate through args[2] when it's just a copy of args[0] in these stages: - Item_func_nullif::walk - Item_func_nullif::update_used_tables I'm not fully sure that skipping args[2] is always correct though. For some transformers it may not be correct. <offtopic> Btw, walk() can be called from the partitioning code when arg_count is still 2 rather than 3. Looks suspicious. Perhaps there is a hidden bug behind that. </offtopic> The remaining heavy piece of the code is: Item* propagate_equal_fields(THD *thd, const Context &ctx, COND_EQUAL *cond) { Context cmpctx(ANY_SUBST, cmp.compare_type(), cmp.compare_collation()); 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]); args[2]->propagate_equal_fields_and_change_item_tree(thd, Context_identity(), cond, &args[2]); return this; } We cannot use the same approach here, because args[0] and args[2] are propagated with a different context (ANY_SUBST vs IDENTITY_SUBST). Not sure what to do with this. Perhaps, data types could have an extra attribute to tell that ANY_SUBST and IDENITTY_SUBST are equal. So, for example, for non-ZEROFILL integers, ANY_SUBST and IDENTITY_SUBST do the same thing and thus propagation could be done one time only. But this would be a partial solution, for a limited number of data types. Another option is to leave propagation as is. Another option is do nothing at all. If we rewrite the reported query as CASE, it will look heavy indeed. So it's Ok to expect heavy execution complexity. Thanks.