Re: Is MaxHeapAttributeNumber a reasonable restriction for foreign-tables?

On Thu, Feb 4, 2021 at 11:45 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Amit Langote <amitlangote09(at)gmail(dot)com> writes:
> > On Thu, Feb 4, 2021 at 4:24 PM Kohei KaiGai <kaigai(at)heterodb(dot)com> wrote:
> >> I think that MaxHeapAttributeNumber is a senseless restriction for foreign-
> >> tables. How about your opinions?
>
> > My first reaction to this was a suspicion that the
> > MaxHeapAttributeNumber limit would be too ingrained in PostgreSQL's
> > architecture to consider this matter lightly, but actually browsing
> > the code, that may not really be the case.
>
> You neglected to search for MaxTupleAttributeNumber...

Ah, I did. Although, even its usage seems mostly limited to modules
under src/backend/access/heap.

> I'm quite skeptical of trying to raise this limit significantly.
>
> In the first place, you'd have to worry about the 2^15 limit on
> int16 AttrNumbers --- and keep in mind that that has to be enough
> for reasonable-size joins, not only an individual table. If you
> join a dozen or so max-width tables, you're already most of the way
> to that limit.
>
> In the second place, as noted by the comment you quoted, there are
> algorithms in various places that are O(N^2) (or maybe even worse?)
> in the number of columns they're dealing with.

Those are certainly intimidating considerations.

--
Amit Langote
EDB: http://www.enterprisedb.com