Re: PATCH: Batch/pipelining support for libpq

On Sat, Mar 11, 2017 at 12:52 AM, Daniel Verite <daniel(at)manitou-mail(dot)org>
wrote:

> Hi,
>
> I notice that PQsetSingleRowMode() doesn't work when getting batch results.
>
> The function is documented as:
> " int PQsetSingleRowMode(PGconn *conn);
>
> This function can only be called immediately after PQsendQuery or one
> of its sibling functions, before any other operation on the connection
> such as PQconsumeInput or PQgetResult"
>
> But PQbatchQueueProcess() unconditionally clears conn->singleRowMode,
> so whatever it was when sending the query gets lost, and besides
> other queries might have been submitted in the meantime.
>
> Also if trying to set that mode when fetching like this
>
> while (QbatchQueueProcess(conn)) {
> r = PQsetSingleRowMode(conn);
> if (r!=1) {
> fprintf(stderr, "PQsetSingleRowMode() failed");
> }
> ..
>
> it might work the first time, but on the next iterations, conn->asyncStatus
> might be PGASYNC_READY, which is a failure condition for
> PQsetSingleRowMode(), so that won't do.
>

Thanks for investigating the problem, and could you kindly explain what
"next iteration" you mean here? Because I don't see any problem in
following sequence of calls - PQbatchQueueProcess(),PQsetSingleRowMode()
, PQgetResult(). Am I missing anything?
Please note that it is MUST to call PQgetResult immediately after
PQbatchQueueProcess() as documented.

>
> ISTM that the simplest fix would be that when in batch mode,
> PQsetSingleRowMode() should register that the last submitted query
> does request that mode, and when later QbatchQueueProcess dequeues
> the batch entry for the corresponding query, this flag would be popped off
> and set as the current mode.
>
> Please find attached the suggested fix, against the v5 of the patch.
>

Before going with this fix, I would like you to consider the option of
asking batch processing users(via documentation) to set single-row mode
before calling PQgetResult().
Either way we need to fix the documentation part, letting users know how
they can activate single-row mode while using batch processing.
Let me know your thoughts.

Best Regards,
Vaishnavi,
Fujitsu Australia.