Re: Status of FDW pushdowns

On Mon, Dec 2, 2013 at 10:26 PM, David Fetter <david(at)fetter(dot)org> wrote:
> On Tue, Dec 03, 2013 at 11:15:36AM +0800, Craig Ringer wrote:
>> On 11/28/2013 03:24 AM, David Fetter wrote:
>> > WITH, or SRF, or whatever, the point is that we need to be able to
>> > specify what we're sending--probably single opaque strings delimited
>> > just as we do other strings--and what we might get back--errors only,
>> > rows, [sets of] refcursors are the ones I can think of offhand.
>>
>> So, you're thinking of something like:
>>
>> WITH FOREIGN somecte AS $$... foreign query ...$$
>> SELECT ...
>> FROM somecte;
>
> I was picturing something a little more like an SRF which would take
> one opaque string, the remote command, some descriptor, perhaps an
> enum, of what if anything might come back. Long ago, I implemented a
> similar thing in DBI-Link. It was called
>
> remote_exec_dbh(data_source_id integer, query text, returns_rows bool)

Couple thoughts:
*) Any 'pass through' API should support parameterization (the FDW may
not support that, but many will and API should allow for it). Lack
of parameterization is a major downside of dblink. The function could
be set up to be variadic for the parameters.

*) For a connectivity APIs of this style, Dblink-ish mechanic of
separating command execution from data returning commands is likely
the right way to go. Also, probably better to stick with SRF
mechanics if we go the 'function route'. So basically we are making
dblink for FDW, adding parameterization and some concept of utilizing
the foreign server.

All this is assuming we are adding a special remote execution function
('fdwlink'). While that would be great, it's a significant deviation
from the standard into postgresql specific SRF syntax. If some of
the qual pushdown deparsing functionality could be put inside the
internal FDW API, then you'd get the best of both worlds. Maybe you'd
still want a dblink style extension anyways, but it wouldn't be as
critical.

merlin