Re: Actual Cost
| От | Jeff Janes |
|---|---|
| Тема | Re: Actual Cost |
| Дата | |
| Msg-id | CAMkU=1zBJNVo2DGYBgLJqpu8fyjCE_ys+msr6pOEoiwA7y5jrA@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Actual Cost (Donald Dong <xdong@csumb.edu>) |
| Ответы |
Re: Actual Cost
Re: Actual Cost Re: Actual Cost |
| Список | pgsql-hackers |
On Sat, Feb 16, 2019 at 10:33 PM Donald Dong <xdong@csumb.edu> wrote:
On Feb 16, 2019, at 6:44 PM, Tomas Vondra wrote:
>
> On 2/17/19 3:40 AM, David Fetter wrote:
>>
>> As someone not volunteering to do any of the work, I think it'd be a
>> nice thing to have. How large an effort would you guess it would be
>> to build a proof of concept?
>
> I don't quite understand what is meant by "actual cost metric" and/or
> how is that different from running EXPLAIN ANALYZE.
Here is an example:
Hash Join (cost=3.92..18545.70 rows=34 width=32) (actual cost=3.92..18500 time=209.820..1168.831 rows=47 loops=3)
Now we have the actual time. Time can have a high variance (a change
in system load, or just noises), but I think the actual cost would be
less likely to change due to external factors.
I don't think there is any way to assign an actual cost. For example how do you know if a buffer read was "actually" seq_page_cost or random_page_cost? And if there were a way, it too would have a high variance.
What would I find very useful is a verbosity option to get the cost estimates expressed as a multiplier of each *_cost parameter, rather than just as a scalar. And at the whole-query level, get an rusage report rather than just wall-clock duration. And if the HashAggregate node under "explain analyze" would report memory and bucket stats; and if the Aggregate node would report...anything.
Cheers,
Jeff
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