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Re: ECPG FETCH readahead

From: Böszörményi Zoltán <zb(at)cybertec(dot)at>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Michael Meskes <meskes(at)postgresql(dot)org>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>, Hans-Juergen Schoenig <hs(at)cybertec(dot)at>
Subject: Re: ECPG FETCH readahead
Date: 2010-06-24 07:27:57
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers

2010-06-23 22:42 keltezéssel, Bruce Momjian írta:
> Boszormenyi Zoltan wrote:
>> Hi,
>> we improved ECPG quite a lot in 9.0 because we worked and
>> still working with an Informix to PostgreSQL migration project.
>> We came across a pretty big performance problem that can be seen in
>> every "naive" application that uses only FETCH 1, FETCH RELATIVE
>> or FETCH ABSOLUTE. These are almost the only FETCH variations
>> usable in Informix, i.e. it doesn't have the grammar for fetching N rows
>> at once. Instead, the Client SDK libraries do caching themselves
>> behind the scenes to reduce network turnaround time.
> I assume our ecpg version supports>1 fetch values, even in Informix
> mode.  Does it make sense to add lots of code to our ecpg then?

I think, yes, it does make sense. Because we are talking
about porting a whole lot of COBOL applications.
The ESQL/C or ECPG connector was already written
the Informix quirks in mind, so it fetches only one record
at a time passing it to the application. And similar performance
is expected from ECPG - which excpectation is not fulfilled
currently because libecpg doesn't do the same caching as
ESQL/C does.

And FYI, I haven't added a whole lot of code, most of the
code changes in the patch is execute.c refactoring.
ECPGdo() was split into several functions, the new parts
are still doing the same things.

I can make the test case much smaller, I only needed
to test crossing the readahead window boundary.
This would also make the patch much smaller.

And this readahead is not on by default, it's only activated
by "ecpg -r fetch_readahead".

Best regards,
Zoltán Böszörményi

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