Overall, I think the direction of the patch set has some promise, but I think it needs a lot of cleanup: removal of unnecessary code, proper formatting, moving variables to inner scopes, explanatory comments, good names for variables and functions and structure members, removal of unnecessary files from the patch, cleanup of the regression test coverage so that it doesn't add more bloat than necessary, proper choice of data structures, and so on. Right now, the good things that you've done here are being hidden by these sorts of mechanical issues. That's not just an issue for me as a reviewer: I suspect it's also blocking you, as the patch author, from finding places where the code could be made better. Being able to find such opportunities for improvement and act on them is what will get this patch from "interesting proof of concept" to "potentially committable patch".
Thanks Robert for your time and attention to this patch.
Based on these review comments and an off list discussion about the design of the patch, I have reworked the patch significantly.
In this version, a tuplestore is added to the PgFdwScanState along with a flag. Now, in case of a cursor switch, this tuplestore is filled with the remaining tuples of the query. The corresponding flag is set to indicate that the tuplestore is ready to be fetched. To remember the last query that was executing, a pointer to its PgFdwScanState is maintained in the conn_state. Note that we only need to remember just the very last query and once tuples for it are fetched we can forget that pointer, because now its fsstate has the remaining tuples in the tuplestore.
So, this version of the patch looks simpler than before.
A few test cases are also added in the attached patch to cover the different scenarios in case of non-cursor mode.
