Re: Can we rely on the ordering of paths in pathlist?

Hi Richard Guo
I tried to changed the comment, can you help me to check if this is
correct?Many thanks.
- /*
- * get_cheapest_parallel_safe_total_inner
- * Find the unparameterized parallel-safe path with the least total cost.
- */
+ /* get_cheapest_parallel_safe_total_inner
+ * Skip paths that do not meet the criteria,find the unparameterized
parallel-safe path with the least total cost and return NULL if it does not
exist.
+ *
+ */

Thanks

wenhui qiu <qiuwenhuifx(at)gmail(dot)com> 于2024年7月31日周三 09:21写道：

> Hi Richard Guo
> Today is the last day of the commitfest cycle.I think this patch
> should be commented ,Except for the comments, I tested it good to me
>
>
> Thanks
>
> wenhui qiu <qiuwenhuifx(at)gmail(dot)com> 于2024年7月25日周四 16:18写道：
>
>> Hi Richard Guo
>> Is it necessary to add some comments here？
>>
>>
>> + if (!innerpath->parallel_safe ||
>> + !bms_is_empty(PATH_REQ_OUTER(innerpath)))
>> + continue;
>> +
>> + if (matched_path != NULL &&
>> + compare_path_costs(matched_path, innerpath, TOTAL_COST) <= 0)
>> + continue;
>> +
>> + matched_path = innerpath;
>>
>> Richard Guo <guofenglinux(at)gmail(dot)com> 于2024年1月10日周三 15:08写道：
>>
>>>
>>> On Tue, Apr 11, 2023 at 11:43 AM Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com>
>>> wrote:
>>>
>>>> On Tue, Apr 11, 2023 at 11:03 AM Richard Guo <guofenglinux(at)gmail(dot)com>
>>>> wrote:
>>>>
>>>>> As the comment above add_path() says, 'The pathlist is kept sorted by
>>>>> total_cost, with cheaper paths at the front.' And it seems that
>>>>> get_cheapest_parallel_safe_total_inner() relies on this ordering
>>>>> (without being mentioned in the comments, which I think we should do).
>>>>>
>>>>
>>>> I think the answer for ${subject} should be yes. Per the comments in
>>>> add_partial_path, we have
>>>>
>>>> * add_partial_path
>>>> *
>>>> * As in add_path, the partial_pathlist is kept sorted with the
>>>> cheapest
>>>> * total path in front. This is depended on by multiple places, which
>>>> * just take the front entry as the cheapest path without searching.
>>>> *
>>>>
>>>
>>> I'm not sure about this conclusion. Surely we can depend on that the
>>> partial_pathlist is kept sorted by total_cost ASC. This is emphasized
>>> in the comment of add_partial_path, and also leveraged in practice, such
>>> as in many places we just use linitial(rel->partial_pathlist) as the
>>> cheapest partial path.
>>>
>>> But get_cheapest_parallel_safe_total_inner works on pathlist not
>>> partial_pathlist. And for pathlist, I'm not sure if it's a good
>>> practice to depend on its ordering. Because
>>>
>>> 1) the comment of add_path only mentions that add_path_precheck relies
>>> on this ordering, but it does not mention that other functions also do;
>>>
>>> 2) other than add_path_precheck, I haven't observed any other functions
>>> that rely on this ordering. The only exception as far as I notice is
>>> get_cheapest_parallel_safe_total_inner.
>>>
>>> On the other hand, if we declare that we can rely on the pathlist being
>>> sorted in ascending order by total_cost, we should update the comment
>>> for add_path to highlight this aspect. We should also include a comment
>>> for get_cheapest_parallel_safe_total_inner to clarify why an early exit
>>> is possible, similar to what we do for add_path_precheck. Additionally,
>>> in several places, we can optimize our code by taking advantage of this
>>> fact and terminate the iteration through the pathlist early when looking
>>> for the cheapest path of a certain kind.
>>>
>>> Thanks
>>> Richard
>>>
>>