Re: postgres_fdw: wrong results with self join + enable_nestloop off

Hi Etsuro,

Thanks for the response!


> Maybe we could do so by leaving to extensions the decision whether
> they replace joins with pseudoconstant clauses, but I am not sure that
> that is a good idea, because that would require the authors to modify
> and recompile their extensions to fix the issue...


I think I cannot easily follow this argument. The decision to push down the
join
(or not) doesn't seem to be related to calling set_join_pathlist_hook. It
seems like the
extension should decide what to do with the hook.

That seems the generic theme of the hooks that Postgres provides. For
example, the extension
is allowed to even override the whole planner/executor, and there is no
condition that would
prevent it from happening. In other words, an extension can easily return
wrong results with the
wrong actions taken with the hooks, and that should be responsibility of
the extension, not Postgres


> I am not familiar with the Citus extension, but such pseudoconstant
> clauses are handled within the Citus extension?
>
>
As I noted earlier, Citus relies on this hook for collecting information
about all the joins that Postgres
knows about, there is nothing specific to pseudoconstants. Some parts of
creating the (distributed)
plan relies on the information gathered from this hook. So, if information
about some of the joins
are not passed to the extension, then the decisions that the extension
gives are broken (and as a result
the queries are broken).

Thanks,
Onder