Subject: Re: logical decoding of two-phase transactions
To: Craig Ringer <craig@2ndquadrant.com>
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Cc: Stas Kelvich <s.kelvich@postgrespro.ru>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
 Robert Haas <robertmhaas@gmail.com>
From: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Message-ID: <e8ac21ca-e0cc-9d97-f6aa-4b9e18d7a4e7@2ndquadrant.com>
Date: Mon, 20 Mar 2017 10:29:44 +0100
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On 20/03/17 09:32, Craig Ringer wrote:
> On 19 March 2017 at 21:26, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote:
> 
>> I think only genam would need changes to do two-phase scan for this as
>> the catalog scans should ultimately go there. It's going to slow down
>> things but we could limit the impact by doing the two-phase scan only
>> when historical snapshot is in use and the tx being decoded changed
>> catalogs (we already have global knowledge of the first one, and it
>> would be trivial to add the second one as we have local knowledge of
>> that as well).
> 
> We'll also have to clobber caches after we finish decoding a 2pc xact,
> since we don't know those changes are visible to other xacts and can't
> guarantee they'll ever be (if it aborts).
> 

AFAIK reorder buffer already does that.

> That's going to be "interesting" when trying to decode interleaved
> transaction streams since we can't afford to clobber caches whenever
> we see an xlog record from a different xact. We'll probably have to
> switch to linear decoding with reordering when someone makes catalog
> changes.

We may need something that allows for representing multiple parallel
transactions in single process and a cheap way of switching between them
(ie, similar things we need for autonomous transactions). But that's not
something current patch has to deal with.

-- 
  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services


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