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
>  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