I would like to enhance the postgres_fdw to allow more complete support for user-defined types.
Right now, postgres_fdw already does a good job of passing user-defined type data back and forth, which is pretty nice.
However,it will not pass functions or operators that use user-defined types to the remote host. For a extension like
PostGIS,that means that spatial filters cannot be executed on remote servers, which makes FDW not so useful for
PostGIS.
I think the postgres_fdw extension should pass user-defined functions and operators, but only when it knows those
functionsand operators exist at the remote. One way would be to ask the remote what extensions it has, but the overhead
ofdoing that is a bit high. A simpler way would be to just have the DBA declare what extensions the remote will have,
whenshe creates the server definition, for example:
CREATE SERVER fire_department_server
FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (
host 'fire.city.gov',
dbname 'infrastructure',
port '5432',
extensions 'postgis, seg'
);
Once the local host knows what extensions to expect on the remote side, it can retain functions and operators in those
extensionsin the set of remote restrictions and deparse them for use in the query of the remote. Basically, everywhere
thereis a call to is_builtin(Oid oid) now, there's also be a call to is_allowed_extension() (or somesuch) as well.
There is a PostGIS-specific implementation of this concept here:
https://github.com/pramsey/postgres/blob/9.4-postgres-fdw-postgis/contrib/postgres_fdw
If the approach above sounds OK, I'll genericize my PostGIS specific code to hand arbitrary extensions and submit a
patch.
Thanks!
Paul
--
Paul Ramsey
http://cleverelephant.ca
http://postgis.net