Re: Disabling options lowers the estimated cost of a query

Hi,

On 4/16/21 3:09 PM, Tom Lane wrote:
> I wrote:
>> ... The code to select the
>> right child path would be approximately like get_cheapest_fractional_path,
>> except that you need to restrict it to paths with the right sort order.
>
> Duh, I forgot about get_cheapest_fractional_path_for_pathkeys().
>
> regards, tom lane
>

The attached patch does fix the issue for me, producing the same plans
with and without partition-wise joins.

It probably needs a bit more work, though:

1) If get_cheapest_fractional_path_for_pathkeys returns NULL, it's not
clear whether to use cheapest_startup or cheapest_total with Sort on
top. Or maybe consider an incremental sort?

2) Same for the cheapest_total - maybe there's a partially sorted path,
and using it with incremental sort on top would be better than using
cheapest_total_path + sort.

3) Not sure if get_cheapest_fractional_path_for_pathkeys should worry
about require_parallel_safe too.

Doesn't seem like an urgent issue (has been there for a while, not sure
we even want to backpatch it). I'll add this to the next CF.

regards

--
Tomas Vondra
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