Re: Actual Cost

On Sat, Feb 16, 2019 at 10:33 PM Donald Dong <xdong(at)csumb(dot)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