Re: Optimization for updating foreign tables in Postgres FDW

On 2016/04/13 3:14, Robert Haas wrote:
> I'm wondering why we are fixing this specific case and not any of the
> other calls to PQexec() or PQexecParams() in postgres_fdw.c.
>
> I mean, many of those instances are cases where the query isn't likely
> to run for very long, but certainly "FETCH %d FROM c%u" is in theory
> just as bad as the new code introduced in 9.6. In practice, it
> probably isn't, because we're probably only fetching 50 rows at a time
> rather than potentially a lot more, but if we're fixing this code up
> to be interrupt-safe, maybe we should fix it all at the same time.
> Even for the short-running queries like CLOSE and DEALLOCATE, it seems
> possible that there could be a network-related hang which you might
> want to interrupt.

Actually, I was wondering, too, but I didn't propose that because, as
far as I know, there are no reports from the field. But I agree with you.

> How about we encapsulate the while (PQisBusy(...)) loop into a new
> function pgfdw_get_result(), which can be called after first calling
> PQsendQueryParams()? So then this code will say dmstate->result =
> pgfdw_get_result(dmstate->conn). And we can do something similar for
> the other call to PQexecParams() in create_cursor(). Then let's also
> add something like pgfdw_exec_query() which calls PQsendQuery() and
> then pgfdw_get_result, and use that to replace all of the existing
> calls to PQexec().
>
> Then all the SQL postgres_fdw executes would be interruptible, not
> just the new stuff.

Seems like a good idea. Will do.

Best regards,
Etsuro Fujita