While re-reviewing the fix, I noticed that since PQcancel we added to pgfdw_xact_callback to cancel a DML pushdown query isn't followed by a ROLLBACK, the connection to the remote server will be discarded at the end of the while loop in that function, which will cause a FATAL error of "connection to client lost". Probably, that was proposed by me in the first version of the patch, but I don't think that's a good idea. Shouldn't we execute ROLLBACK after that PQcancel?

Another thing I noticed is, ISTM that we miss the case where DML pushdown queries are performed in subtransactions. I think cancellation logic would also need to be added to pgfdw_subxact_callback.

Comments are welcome!

Best regards,
Etsuro Fujita

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