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Re: extension facility (was: revised hstore patch)

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Dimitri Fontaine <dfontaine(at)hi-media(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: extension facility (was: revised hstore patch)
Date: 2009-07-22 20:11:59
Message-ID: 603c8f070907221311m1e33f742la3c6e3cd401cdda8@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Jul 21, 2009 at 8:56 PM, Robert Haas<robertmhaas(at)gmail(dot)com> wrote:
> A decent module infrastructure is probably not going to fix this
> problem unless it links with -ldwiw.  There are really only two
> options here:
>
> - Keep the old version around for compatibility and add a new version
> that isn't compatible, plus provide a migration path from the old
> version to the new version.
>
> - Make the new version read the format written by the old version.

On Wed, Jul 22, 2009 at 1:40 PM, Dimitri Fontaine<dfontaine(at)hi-media(dot)com> wrote:
> I beg to defer. The way for a decent *extension* facility to handle the case
> is by providing an upgrade function which accepts too arguments: old and new
> version of the module. Then the module author is able to run custom code
> from within the module upgrade transaction, where migrating on disk data
> representation is entirely possible. pg_depend would have to allow for easy
> finding of a given datatype column I guess.

Technically I suppose you're right,  but I don't think that's going to
work very well in practice.  The whole point of binary upgrade is that
you have a really big database where dump and reload is not practical.
 The numbers we've seen for pg_migrator make copy mode look WAY slower
than link mode, and that's without doing any transformation on the
data.

If you make the new code read the old on-disk format, you can upgrade
your data a bit at a time if you want to.  You can, for example, write
a script to do a no-op update on a few thousand tuples every hour
until they're all updated.  That's really important for big datasets.

If you keep an old and a new version of the datatype, you can't
upgrade a tuple at a time, but you can at least upgrade one column at
a time, which is still better than a kick in the head.

If you make the extension-upgrade facility rewrite everything, you
have to do your entire cluster in one shot.  That will work for some
people, but not for all.  And unless you ship both versions of hstore
with either PG 8.4 or PG 8.5, you're going to need the conversion to
be done inside pg_migrator, which introduces a whole new level of
complexity that I think we'd be better off without.

...Robert

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