Thread

Re: MergeAppend could consider sorting cheapest child path

Alexander Pyhalov <a.pyhalov@postgrespro.ru> — 2025-05-07T09:06:05Z

Andrei Lepikhov писал(а) 2025-05-07 12:03:
> On 7/5/2025 08:57, Alexander Pyhalov wrote:
>> Andrei Lepikhov писал(а) 2025-05-07 08:02:
>>> On 5/5/2025 15:56, Alexander Pyhalov wrote:
>>>> Andrei Lepikhov писал(а) 2025-05-05 14:38:
>>>> Also logic a bit differs if path is NULL. In 
>>>> get_cheapest_path_for_pathkeys_ext() we explicitly check for path 
>>>> being NULL, in get_cheapest_fractional_path_for_pathkeys_ext() only 
>>>> after calculating sort cost.
>>>> 
>>>> I've tried to fix comments a bit and unified functions definitions.
>>> Generally seems ok, I'm not a native speaker to judge the comments. 
>>> But:
>>> if (base_path && path != base_path)
>>> 
>>> What is the case in your mind where the base_path pointer still may 
>>> be null at that point?
>> 
>> Hi.
>> 
>> It seems if some childrel doesn't have valid pathlist, subpaths_valid 
>> would be false in add_paths_to_append_rel()
>> and generate_orderedappend_paths() will not  be called. So when we 
>> iterate over live_childrels,
>> all of them will have cheapest_total path. This is why we can assert 
>> that base_path is not NULL.
> I'm not sure I understand you correctly. Under which conditions will 
> rel->cheapest_total_path be set to NULL for a childrel? Could you 
> provide an example?

Sorry, perhaps I was not clear enough. I've stated the opposite - it 
seems we can be sure that it's not NULL.
-- 
Best regards,
Alexander Pyhalov,
Postgres Professional