Re: pgsql_fdw, FDW for PostgreSQL server

Tom Lane wrote:
> Shigeru Hanada <shigeru(dot)hanada(at)gmail(dot)com> writes:
>> (2011/12/12 22:59), Robert Haas wrote:
>>> ... I feel like we might need a system here that
>>> allows for more explicit user control about what to push down vs.
not,
>>> rather than assuming we'll be able to figure it out behind the
scenes.

>> Agreed. How about to add a per-column boolean FDW option, say
>> "pushdown", to pgsql_fdw? Users can tell pgsql_fdw that the column
can
>> be pushed down safely by setting this option to true.

> [ itch... ] That doesn't seem like the right level of granularity.
> ISTM the problem is with whether specific operators have the same
> meaning at the far end as they do locally. If you try to attach the
> flag to columns, you have to promise that *every* operator on that
> column means what it does locally, which is likely to not be the
> case ever if you look hard enough. Plus, having to set the flag on
> each individual column of the same datatype seems pretty tedious.
>
> I don't have a better idea to offer at the moment though. Trying
> to attach such a property to operators seems impossibly messy too.
> If it weren't for the collations issue, I might think that labeling
> datatypes as being compatible would be a workable approximation.

Maybe I'm missing something, but if pushdown worked as follows:

- Push down only system functions and operators on system types.
- Only push down what is guaranteed to work.

then the only things we would miss out on are encoding- or
collation-sensitive string operations.

Is that loss so big that it warrants a lot of effort?

Yours,
Laurenz Albe