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Re: Incremental results from libpq

From: "Goulet, Dick" <DGoulet(at)vicr(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>,"Peter Eisentraut" <peter_e(at)gmx(dot)net>
Cc: <pgsql-interfaces(at)postgresql(dot)org>,"Scott Lamb" <slamb(at)slamb(dot)org>
Subject: Re: Incremental results from libpq
Date: 2005-11-16 14:57:27
Message-ID: 4001DEAF7DF9BD498B58B45051FBEA6502EF559C@25exch1.vicorpower.vicr.com (view raw or flat)
Thread:
Lists: pgsql-interfaces
Tom,

	Your case for not supporting this is reasonable, at least to me.
Personally I believe you should take one side or the other at the server
level and then allow the app developer to use it as appropriate, so no
argument here.  But, there was a change in behavior introduced by Oracle
in 10G that supports what was asked for by Trolltech.  The optimizer was
provided the "smarts" to determine if your query is best supported by a
regular cursor or if a bulk collect in the background would be better.
The end result is that the application behaves as normal, but the
results are faster at getting back to it.  What appears to be happening
is that the database returns the first row as normal, but then continues
collecting data rows and sequestering then off some where, probably the
temp tablespace, until your ready for them.  Appears to have driven the
final coffin nail in the old "ORA-01555 Snapshot too old" error.  Course
since Postgresql doesn't have undo segments you don't have that problem.

-----Original Message-----
From: pgsql-interfaces-owner(at)postgresql(dot)org
[mailto:pgsql-interfaces-owner(at)postgresql(dot)org] On Behalf Of Tom Lane
Sent: Wednesday, November 16, 2005 9:24 AM
To: Peter Eisentraut
Cc: pgsql-interfaces(at)postgresql(dot)org; Scott Lamb
Subject: Re: [INTERFACES] Incremental results from libpq 

Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> Am Mittwoch, 9. November 2005 22:22 schrieb Tom Lane:
>> The main reason why libpq does what it does is that this way we do
not
>> have to expose in the API the notion of a command that fails part way
>> through.

> I'm at LinuxWorld Frankfurt and one of the Trolltech guys came over to
talk to 
> me about this.  He opined that it would be beneficial for their
purpose (in 
> certain cases) if the server would first compute the entire result set
and 
> keep it in the server memory (thus eliminating potential errors of the
1/x 
> kind) and then ship it to the client in a way that the client would be
able 
> to fetch it piecewise.  Then, the client application could build the
display 
> incrementally while the rest of the result set travels over the (slow)
link.  
> Does that make sense?

Ick.  That seems pretty horrid compared to the straight
incremental-compute-and-fetch approach.  Yes, it preserves the illusion
that a SELECT is all-or-nothing, but at a very high cost, both in terms
of absolute runtime and in terms of needing a new concept in the
frontend protocol.  It also doesn't solve the problem for people who
need incremental fetch because they have a result set so large they
don't want it materialized on either end of the wire.  Furthermore, ISTM
that any client app that's engaging in incremental fetches really has to
deal with the failure-after-part-of-the-query-is-done problem anyway,
because there's always a risk of failures on the client side or in the
network connection.  So I don't see any real gain in conceptual
simplicity from adding this feature anyway.

Note that if Trolltech really want this behavior, they can have it today
--- it's called CREATE TEMP TABLE AS SELECT.  It doesn't seem attractive
enough to me to justify any further feature than that.

			regards, tom lane

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