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Re: Catalog/Metadata consistency during changeset extraction from wal

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Andres Freund" <andres(at)2ndquadrant(dot)com>, "Robert Haas" <robertmhaas(at)gmail(dot)com>
Cc: "Simon Riggs" <simon(at)2ndquadrant(dot)com>, "Florian Pflug" <fgp(at)phlo(dot)org>,<pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Catalog/Metadata consistency during changeset extraction from wal
Date: 2012-06-25 18:50:54
Message-ID: 4FE86CBE0200002500048AB2@gw.wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-hackers
Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
 
>> We most particularly *don't* want DDL to replicate automatically,
>> because the schema changes are deployed along with related
>> software changes, and we like to pilot any changes for at least a
>> few days.  Depending on the release, the rollout may take a
>> couple months, or we may slam in out everywhere a few days after
>> the first pilot deployment.
> Thats a sensible for your use-case - but I do not think its thats
> the appropriate behaviour for anything which is somewhat
> out-of-the box...
 
Right.  We currently consider the issues involved in a change and
script it by hand.  I think we want to continue to do that.  The
point was that, given the variety of timings and techniques for
distributing schema changes, maybe is was only worth doing that
automatically for the most constrained and controlled cases.
 
>> So you could certainly punt all of this for any release as far as
>> Wisconsin Courts are concerned.  We need to know table and column
>> names, before and after images, and some application-supplied
>> metadata.
> I am not sure were going to get all that into 9.3.
 
Sure, that was more related to why I was questioning how much these
use cases even *could* integrate -- whether it even paid to
*consider* these use cases at this point.  Responses from Robert and
you have pretty much convinced me that I was being overly
pessimistic on that.
 
One fine point regarding before and after images -- if a value
doesn't change in an UPDATE, there's no reason to include it in both
the BEFORE and AFTER tuple images, as long as we have the null
column bitmaps -- or some other way of distinguishing unchanged from
NULL.  (This could be especially important when the unchanged column
was a 50 MB bytea.)  It doesn't matter to me which way this is
optimized -- in our trigger-based system we chose to keep the full
BEFORE tuple and just show AFTER values which were distinct from the
corresponding BEFORE values, but it would be trivial to adapt the
code to the other way.
 
Sorry for that bout of pessimism.
 
-Kevin

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