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Re: Speed dblink using alternate libpq tuple storage

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Marko Kreen <markokr(at)gmail(dot)com>
Cc: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)oss(dot)ntt(dot)co(dot)jp>, greg(at)2ndquadrant(dot)com, pgsql-hackers(at)postgresql(dot)org, mmoncure(at)gmail(dot)com, shigeru(dot)hanada(at)gmail(dot)com
Subject: Re: Speed dblink using alternate libpq tuple storage
Date: 2012-03-30 22:13:50
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Lists: pgsql-hackers
Marko Kreen <markokr(at)gmail(dot)com> writes:
> On Fri, Mar 30, 2012 at 05:18:42PM -0400, Tom Lane wrote:
>> I'm pretty dissatisfied with the error reporting situation for row
>> processors.  You can't just decide not to solve it, which seems to be
>> the current state of affairs.  What I'm inclined to do is to add a
>> "char **" parameter to the row processor, and say that when the
>> processor returns -1 it can store an error message string there.

> But such API seems to require specifying allocator, which seems ugly.

Not if the message is a constant string, which seems like the typical
situation (think "out of memory").  If the row processor does need a
buffer for a constructed string, it could make use of some space in its
"void *param" area, for instance.

> I think it would be better to just use Kyotaro's original idea
> of PQsetRowProcessorError() which nicer to use.

I don't particularly care for that idea because it opens up all sorts of
potential issues when such a function is called at the wrong time.
Moreover, you have to remember that the typical situation here is that
we're going to be out of memory or otherwise in trouble, which means
you've got to be really circumspect about what you assume will work.
Row processors that think they can do a lot of fancy message
construction should be discouraged, and an API that requires
construction of a new PGresult in order to return an error is right out.
(This is why getAnotherTuple is careful to clear the failed result
before it tries to build a new one.  But that trick isn't going to be
available to an external row processor.)

			regards, tom lane

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Subject: Re: Speed dblink using alternate libpq tuple storage
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