Re: PATCH: Batch/pipelining support for libpq

From: "Daniel Verite" <daniel(at)manitou-mail(dot)org>
To: "Vaishnavi Prabakaran" <vaishnaviprabakaran(at)gmail(dot)com>
Cc: "Michael Paquier" <michael(dot)paquier(at)gmail(dot)com>,"Prabakaran, Vaishnavi" <VaishnaviP(at)fast(dot)au(dot)fujitsu(dot)com>,"Craig Ringer" <craig(at)2ndquadrant(dot)com>,"Haribabu Kommi" <kommi(dot)haribabu(at)gmail(dot)com>,"Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>,"Dmitry Igrishin" <dmitigr(at)gmail(dot)com>,"Andres Freund" <andres(at)anarazel(dot)de>,"PostgreSQL Hackers" <pgsql-hackers(at)postgresql(dot)org>,"Manuel Kniep" <m(dot)kniep(at)web(dot)de>,"fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp" <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>,"Iwata, Aya" <iwata(dot)aya(at)jp(dot)fujitsu(dot)com>
Subject: Re: PATCH: Batch/pipelining support for libpq
Date: 2017-03-10 13:52:05
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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.

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.

Best regards,
Daniel Vérité
PostgreSQL-powered mailer:
Twitter: @DanielVerite

Attachment Content-Type Size
fix-for-singlerowmode.diff text/x-patch 2.0 KB

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