Re: Top-N sorts verses parallelism

On Tue, Dec 19, 2017 at 5:24 PM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> On Mon, Dec 18, 2017 at 9:29 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> I went through the callers to create_sort_path and the only one that
>> looks like it can pass a limit is the one you and Jeff already
>> identified. So I think the question is just whether
>> create_gather_merge_path needs a similar fix.
>
> I might be missing something, but it looks like there are no cases
> where we have a limit_tuples value we could use AND we're relying on
> create_gather_merge_path's own ability to create sort paths. So I
> suspect there is no reason to change create_gather_merge_path itself
> to deal with tuple limits.
>

Exactly. I was about to post the same and my analysis results are
same as yours. I think this raises the question, do we really need
cost_sort at that place and if so for which case?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com