From: | Boszormenyi Zoltan <zb(at)cybertec(dot)at> |
---|---|
To: | Noah Misch <noah(at)leadboat(dot)com>, Michael Meskes <meskes(at)postgresql(dot)org>, Robert Haas <robertmhaas(at)gmail(dot)com>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us> |
Subject: | Re: ECPG FETCH readahead |
Date: | 2012-04-16 04:24:57 |
Message-ID: | 4F8B9F19.3040403@cybertec.at |
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2012-04-16 04:46 keltezéssel, Michael Meskes írta:
> On Tue, Apr 10, 2012 at 07:56:35PM +0200, Boszormenyi Zoltan wrote:
>> With the above, it would be possible to use a comma separated list of "-r"
>> suboptions, e.g. "-r prepare,questionmarks,readahead=16" in one option.
> Yes, that sounds like a good plan. But of course it's outside the scope of this
> patch, so we can add this later on.
>
>> - Also added a note to the documentation about a possible performance trap
>> if a previously written ECPG application uses its own custom readahead via
>> multi-row FETCH statements.
> I didn't know that before you send this patch. Noah, did you?
>
> Frankly, I don't like this at all. If I got it right that means a FETCH N is
> essantially computed as N times FETCH 1 unless you either add a non-standard
> option to the DECLARE statement or you add a command-line option to ecpg. Did I
> get that right?
Yes, just like when the readahead window set to 256, FETCH 1024
will iterate through 4 windows or FETCH 64 iterates through the
same window 4 times. This is the idea behind the "readahead window".
> If so we would deliberately make ecpglib work incorrectly and remove
> performance. Why is that? I'm interested in what others think, but to me that
> sounds like a show-stopper.
How about allowing the readahead window to be resized for the
non-decorated case if the runtime encounters FETCH N and N is
greater than the previous window?
>
> Michael
>
--
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Zoltán Böszörményi
Cybertec Schönig& Schönig GmbH
Gröhrmühlgasse 26
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Web: http://www.postgresql-support.de
http://www.postgresql.at/
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