Re: [PATCH 4/4] Add tests to dblink covering use of COPY TO FUNCTION

Robert Haas wrote:
> I'm fuzzy on what problem this is attempting to solve... as mentioned
> in the above guidelines, it's usually good to start with some design
> discussions before writing/submitting code.
This has been through some heavy design discussions with a few PG
hackers you know and some you don't, they just couldn't release the
result until now. As for what it's good for, if you look at what you
can do now with dblink, you can easily move rows between nodes using
dblink_build_sql_insert. This is perfectly fine for small bits of work,
but the performance isn't nearly good enough to do serious replication
with it. The upper level patch here allows using COPY as the mechanism
to move things between them, which is much faster for some use cases
(which includes most of the really useful ones). It dramatically
increases the scale of what you can move around using dblink as the
replication transport.

The lower level patch is needed to build that layer, which is an
immediate proof of its utility. In addition, adding a user-defined
function as a COPY target opens up all sorts of possibilities for things
like efficient ETL implementation. And if this approach is accepted as
a reasonable one, as Dan suggested a next step might even be to
similarly allow passing COPY FROM through a UDF, which has the potential
to provide a new efficient implementation path for some of the custom
input filter requests that pop up here periodically.

--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com