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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 21:18:42
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Marko Kreen <markokr(at)gmail(dot)com> writes:
> On Wed, Mar 07, 2012 at 03:14:57PM +0900, Kyotaro HORIGUCHI wrote:
>>> My suggestion - check in getAnotherTuple whether resultStatus is
>>> already error and do nothing then.  This allows internal pqAddRow
>>> to set regular "out of memory" error.  Otherwise give generic
>>> "row processor error".

>> Current implement seems already doing this in
>> parseInput3(). Could you give me further explanation?

> The suggestion was about getAnotherTuple() - currently it sets
> always "error in row processor".  With such check, the callback
> can set the error result itself.  Currently only callbacks that
> live inside libpq can set errors, but if we happen to expose
> error-setting function in outside API, then the getAnotherTuple()
> would already be ready for it.

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.
If it does so, that's what we report.  If it doesn't (which we'd detect
by presetting the value to NULL), then use a generic "error in row
processor" message.  This is cheap and doesn't prevent the row processor
from using some application-specific error reporting method if it wants;
but it does allow the processor to make use of libpq's error mechanism
when that's preferable.

			regards, tom lane

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