Re: Custom FDW - the results of a nested query/join not being passed as qual to the outer query

Many thanks for the fast response.

Using an SRF is an interesting idea, I'll have a play and see if we can
make that work.

Cheers,
Kai

On Wed, Jan 27, 2021 at 3:27 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Kai Daguerre <kai(at)turbot(dot)com> writes:
> > We often have virtual tables where a list operation is not
> viable/possible
> > without providing quals. For example we have implemented a 'whois' table,
> > which will retrieve whois information for specified domains. It is
> clearly
> > not practical to do an unqualified 'list' of *all* domains.
>
> In that case you're going to have to resign yourself to some queries
> failing. This is unavoidable, consider "select * from whois". But
> just because the query has a WHERE condition doesn't mean that a useful
> restriction clause can be extracted for any particular table.
>
> I think the best you can do is (1) fail at runtime if there's no qual
> and (2) at plan time, return an extremely high cost estimate for a
> qual-less scan, in hopes of discouraging the planner from choosing
> that. (Instead of (2), you could perhaps not generate a scan path
> at all, but that's likely to lead to an unintelligible error message.)
>
> Perhaps you should rethink whether you really want a foreign table
> rather than a set-returning function. With the SRF approach it's
> automatic that the user must supply the restricting argument(s) you need.
>
> regards, tom lane
>