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

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