Message-ID: <4F8B9F19.3040403@cybertec.at>
Date: Mon, 16 Apr 2012 06:24:57 +0200
From: Boszormenyi Zoltan <zb@cybertec.at>
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To: Noah Misch <noah@leadboat.com>, Michael Meskes <meskes@postgresql.org>,
	Robert Haas <robertmhaas@gmail.com>,
	PG Hackers <pgsql-hackers@postgresql.org>,
	Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>,
	Bruce Momjian <bruce@momjian.us>
Subject: Re: ECPG FETCH readahead
References: <4F74E6A7.8040204@cybertec.at>
	<20120407112008.GA2286@feivel.credativ.lan>
	<20120407155042.GB11987@tornado.leadboat.com>
	<20120408142501.GA21513@feivel.credativ.lan>
	<4F81BE55.1010904@cybertec.at>
	<20120408173859.GA4915@feivel.credativ.lan>
	<4F83E2B9.3030302@cybertec.at>
	<20120410143722.GC6129@tornado.leadboat.com>
	<20120410145501.GA11554@feivel.credativ.lan>
	<4F847453.9060200@cybertec.at>
	<20120416024636.GA4536@feivel.credativ.lan>
In-Reply-To: <20120416024636.GA4536@feivel.credativ.lan>
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2012-04-16 04:46 keltez=C3=A9ssel, Michael Meskes =C3=ADrta:
> 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=3D16" in one opti=
on.
> 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 readah=
ead 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-sta=
ndard
> option to the DECLARE statement or you add a command-line option to ecp=
g. 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 m=
e 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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