Re: SQL/MED - core functionality

Thanks for the comments.

I'll revise the patch along the discussion.  Before starting code work,
please let me summarize the discussion.

* Generally, we should keep FDWs away from PostgreSQL internals,
such as TupleTableSlot.

* FDW should have planner hook which allows FDW to create FDW-specific
plan (FdwPlan in Heikki's proposal) for a scan on a foreign table.

* FdwPlan, a part of ForeignScan plan node, should be able to be
copied in generic way because plans would be copied into another
memory context during caching.  It might be better to represent
FdwPlan with Node or List.

* FdwExecutionState, a part of ForeignScanState, should be used
instead of ForeignScanState to remove executor details from FDW
implementation.
# ISTM that FdwExecutionState would be replace FdwReply.

Regards,
--
Shigeru Hanada