There was a discussion Some Time Ago about how to accomplish an
upgrade from 1.2 to 2.0, where I outlined a proposed mechanism.

  http://lists.slony.info/pipermail/slony1-general/2009-February/009193.html

It bears a certain resemblance to pg_migrator in the sense that it
would do rather a lot of grovelling around throughout the Slony-I data
structures to fix things up.

Another thought came up this week; Jan suggested the thought that what
we could do is to add an OMIT COPY option to the SUBSCRIBE SET
command, and basically handle the upgrade via the following process:

  Step 0.  You decide it's time to upgrade.

           Lock out users (via pg_hba.conf), and make sure replication
           is caught up.

           Outage of application begins.

            ---- From here on, it's important for there to be ZARRO
                 changes made to data!!! ----
  
  Step 1.  Stop v1.2 slons.

  Step 2.  Run UNINSTALL NODE on all nodes.

           At this point, replication is gone from your environment,
           and all nodes are recovered so that pg_catalog is no longer
           "hacked up" by Slony-I.

  Step 3.  You may wish to stop postmaster as part of the process of
           shuffling new Slony-I 2.0 binaries into place.

  Step 4.  Recreate replication.

           Using Slony-I 2.0 tools.

           The notable Special Thing is that SUBSCRIBE SET requests
           are submitted with the new OMIT COPY option.

           This option causes the slons to not bother doing
           TRUNCATE/COPY.

Steps 1-4 should only take a few seconds, as:
 a) Everyone's locked out, so there are no locks being held
 b) We're just throwing in metadata, NOT 18-hour-long COPY statements

Once all nodes are subscribed, and things are looking well, it should
be safe to let the users back in, and start back up.

There are a number of other possible uses for SUBSCRIBE SET with OMIT
COPY; I'll leave thoughts about that to other postings :-).

I have a seemingly-working patch, along with a regression test;
attached below.

Comments, rocks, tomatoes, all welcome :-)

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