Re: Ordered Partitioned Table Scans

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Fri, Mar 22, 2019 at 11:56 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> In cases where, say, the first child requires no sort but also doesn't
>> emit very many rows, while the second child requires an expensive sort,
>> the planner will have a ridiculously optimistic opinion of the cost of
>> fetching slightly more rows than are available from the first child.
>> This might lead it to wrongly choose a merge join over a hash for example.

> I think this is very much a valid point, especially in view of the
> fact that we already choose supposedly fast-start plans too often. I
> don't know whether it's a death sentence for this patch, but it should
> at least make us stop and think hard.

Once again: this objection is not a "death sentence for this patch".
I simply wish to suppress the option to generate an ordered Append
when some of the inputs would require an added sort step. As long
as we have pre-ordered paths for all children, go for it.

regards, tom lane