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Re: Multiple queries in transit

From: Merlin Moncure <mmoncure(at)gmail(dot)com>
To: Mark Hills <Mark(dot)Hills(at)framestore(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Multiple queries in transit
Date: 2011-10-31 17:53:10
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Lists: pgsql-hackers
On Mon, Oct 31, 2011 at 12:49 PM, Mark Hills <Mark(dot)Hills(at)framestore(dot)com> wrote:
> On Mon, 31 Oct 2011, Tom Lane wrote:
>> Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>> > On 31.10.2011 17:44, Mark Hills wrote:
>> >> Could libpq be reasonably modified to allow this?
>> > I believe it's doable in theory, no-one has just gotten around to it.
>> > Patches are welcome.
>> Can't you do that today with a multi-command string submitted to
>> PQsendQuery, followed by multiple calls to PQgetResult?
> I remember something about this; I think I concluded that it validated
> that receiving multiple results could be done this way.
> But this kind of batching can't be used with prepared queries.
>> I'm hesitant to think about supporting the case more thoroughly than
>> that, or with any different semantics than that, because I think that
>> the error-case behavior will be entirely unintelligible/unmaintainable
>> unless you abandon all queries-in-flight in toto when an error happens.
> Can you explain a bit more detail which errors are of most concern, do you
> mean full buffers on the client send?
> Because the content of the stream going to/from the server does not
> change, I wouldn't really expect the semantics to change. For example, the
> server cannot even see that the client is behaving in this way. Are there
> any 'send' functions that are heavily reliant on some kind of
> result/receive state?
> I don't disagree with the comments above though, any shift towards
> unintelligible behaviour would be very bad.
>> Furthermore, in most apps it'd be a serious PITA to keep track of which
>> reply is for which query, so I doubt that such a feature is of general
>> usefulness.
> In our UI case, we already have a queue. Because libpq can't pipeline
> multiple queries, we have to make our own queue of them anyway.

Note, nothing is keeping you from opening up a second connection and
interleaving in that fashion, so 'libpq' is not the bottleneck, the
connection object is :-).


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