On Wed, Mar 2, 2016 at 6:54 PM, Alexander Korotkov
<a.korot...@postgrespro.ru> wrote:
> If FDWs existed then Postgres XC/XL were being developed then I believe they
> would try to build full-featured prototype of FDW based sharding. If this
> prototype succeed then we could make a full roadmap.

Speaking here with my XC hat, that's actually the case. A couple of
years back when I worked on it, there were discussions about reusing
FDW routines for the purpose of XC, which would have been roughly
reusing postgres_fdw + the possibility to send XID, snapshot and
transaction timestamp to the remote nodes after getting that from the
GTM (global transaction manager ensuring global data visibility and
consistency), and have the logic for query pushdown in the FDW itself
when planning query on what would have been roughly foreign tables
(not entering in the details here, those would have not been entirely
foreign tables). At this point the global picture was not completely
set, XC being based on 9.1~9.2 and the FDW base routines were not as
extended as they are now. As history has told, this global picture has
never showed up, though it would should XC have been merged with 9.3.
The point is that XC would have moved as using the FDW approach, as a
set of plugins.

This was a reason behind this email of 2013 on -hackers actually:

There were as well discussions about making the connection pooler a
background worker and plug in that in a shared memory context that all
backends connecting to this XC-like-postgres_fdw would use, though
this is another story, for another time...

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