Re: Optimization for updating foreign tables in Postgres FDW

On 2016/04/19 13:59, Michael Paquier wrote:
> On Tue, Apr 19, 2016 at 1:14 PM, Etsuro Fujita
> <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> What do you think about that?

> + /* Wait for the result */
> + res = pgfdw_get_result(conn, query);
> + if (res == NULL)
> + pgfdw_report_error(ERROR, NULL, conn, false, query);
> + last_res = res;
> +
> + /*
> + * Verify that there are no more results
> + *
> + * We don't use a PG_TRY block here, so be careful not to throw error
> + * without releasing the PGresult.
> + */
> + res = pgfdw_get_result(conn, query);
> + if (res != NULL)
> + {
> + PQclear(last_res);
> + pgfdw_report_error(ERROR, res, conn, true, query);
> + }
>
> But huge objection to that because this fragilizes the current logic
> postgres_fdw is based on: PQexec returns the last result to caller,
> I'd rather not break that logic for 9.6 stability's sake.

IIUC, I think each query submitted by PQexec in postgres_fdw.c contains
just a single command. Maybe I'm missing something, though.

> A even better proof of that is the following, which just emulates what
> your version of pgfdw_get_result is doing when consuming the results.
> + /* Verify that there are no more results */
> + res = pgfdw_get_result(fmstate->conn, fmstate->query);
> + if (res != NULL)
> + pgfdw_report_error(ERROR, res, fmstate->conn, true, fmstate->query);
> This could even lead to incorrect errors in the future if multiple
> queries are combined with those DMLs for a reason or another.

I'd like to leave such enhancements for future work...

Thanks for the comment!

Best regards,
Etsuro Fujita