Re: Startup cost of sequential scan

On 30.08.2018 17:58, Tom Lane wrote:
> Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru> writes:
>> On Thu, Aug 30, 2018 at 5:05 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Because it's what the mental model of startup cost says it should be.
>> From this model we make a conclusion that we're starting getting rows
>> from sequential scan sooner than from index scan. And this conclusion
>> doesn't reflect reality.
> No, startup cost is not the "time to find the first row". It's overhead
> paid before you even get to start examining rows.
But it seems to me that calculation of cost in LIMIT node contradicts
with this statement:

            pathnode->path.startup_cost +=
                (subpath->total_cost - subpath->startup_cost)
                * offset_rows / subpath->rows;

>
> I'm disinclined to consider fundamental changes to our costing model
> on the basis of this example. The fact that the rowcount estimates are
> so far off reality means that you're basically looking at "garbage in,
> garbage out" for the cost calculations --- and applying a small LIMIT
> just magnifies that.
>
> It'd be more useful to think first about how to make the selectivity
> estimates better; after that, we might or might not still think there's
> a costing issue.
>
> regards, tom lane
>

--
Konstantin Knizhnik
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