From: Tom Lane <tgl@sss.pgh.pa.us>
To: Steve Midgley <science@misuse.org>
cc: Kirmo Uusitalo <kirmo.uusitalo@gmail.com>,
        Samed YILDIRIM <samed@reddoc.net>, pgsql-sql@lists.postgresql.org
Subject: Re: Postgresql long transaction support
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Comments: In-reply-to Steve Midgley <science@misuse.org>
	message dated "Wed, 14 Dec 2022 11:59:44 -0800"
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Date: Wed, 14 Dec 2022 16:13:32 -0500
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Steve Midgley <science@misuse.org> writes:
> This seems pretty complex to address at any layer! But it would be a useful
> feature for some situations, for sure. It seems like the approach taken by
> Ruby/Rails and similar frameworks for "db migrations" might have some
> useful design patterns for you. Each transaction has a link to its future
> and past transaction. If the system wants to traverse from "migration 6 to
> migration 3" and the DB is currently at migration 6 (stored a version
> number in the db itself), it runs the exit function to downgrade to
> migration 5, then runs the logic for migration 5, then runs the downgrade
> to 4, etc.

> It can be slow, as implemented, but it allows you to attach logic to every
> step forward and backward in time that ensures you can handle even
> structure changes in the tables, etc (as well as changing lookup values in
> tables, etc).

> Might be worth considering as a design model?

If you end up building things that way, PG's "extensions" could be a
useful way to package it.  What Steve is calling a "migration" corresponds
to an extension version, and you can provide scripts to go in either
direction between versions.

			regards, tom lane