Re: Planner doesn't look at LIMIT?

On 7/22/05, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > This is quite strange. The nestloop plan definitely should be preferred
> > in the context of the LIMIT, considering that it has far lower estimated
> > cost. And it is preferred in simple tests for me.
>
> After a suitable period of contemplating my navel, I figured out
> what is going on here: the total costs involved are large enough that
> the still-fairly-high startup cost of the hash is disregarded by
> compare_fuzzy_path_costs(), and so the nestloop is discarded as not
> having any significant potential advantage in startup time.
>
> I think that this refutes the original scheme of using the same fuzz
> factor for both startup and total cost comparisons, and therefore
> propose the attached patch.
>
> Comments?

Works great!!!

With LIMIT below 4 000 000 rows (its 47-milion row table) it prefers
nested loops, then it starts to introduce merge joins.

Regards,
Dawid