Subject: Re: [HACKERS] logical decoding of two-phase transactions
To: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>
Cc: Nikhil Sontakke <nikhils@2ndquadrant.com>,
 David Steele <david@pgmasters.net>, Andres Freund <andres@anarazel.de>,
 Andrew Dunstan <andrew.dunstan@2ndquadrant.com>,
 Simon Riggs <simon@2ndquadrant.com>, Craig Ringer <craig@2ndquadrant.com>,
 Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
 Petr Jelinek <petr.jelinek@2ndquadrant.com>,
 Sokolov Yura <y.sokolov@postgrespro.ru>,
 Stas Kelvich <s.kelvich@postgrespro.ru>,
 Dmitry Dolgov <9erthalion6@gmail.com>,
 Masahiko Sawada <sawada.mshk@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>
References: <782517ca-1e99-06fe-eba9-34f826118289@2ndquadrant.com>
 <9cac2837-7e58-1cad-3940-27ac0dd7c198@2ndquadrant.com>
 <CAMGcDxfAnJtJ17hejKFDa3xXS2OWpys+p-PvGS+MZj4h73QqPA@mail.gmail.com>
 <b1c6ba51-436c-b10d-895f-3805d3caf4a5@2ndquadrant.com>
 <CAMGcDxeyO-vu4WEsX8ZZRkV7LB037Rd5EmekQuZ_bj0Y33KbBA@mail.gmail.com>
 <CAMGcDxeW2xE51Rx8V8hwL7YMkVFBwtOwDco5=opWd-YACJMi0g@mail.gmail.com>
 <CAA8=A7_c9jiNMzpHsH+wxEA91WUt_cRc-BN8U0S+MErhJnhZTw@mail.gmail.com>
 <20180406162023.wcic5kw73bgswgb6@alap3.anarazel.de>
 <CAMGcDxev9QiGpAkrNxvTKeGtYJTKRmo0UxJuQUyWGuOKgjEtdw@mail.gmail.com>
 <9ea4c35b-f8ec-220b-2429-b860b49748f3@pgmasters.net>
 <CAMGcDxfmy1nLtoukiHbavfqm+scvT9=WckaCtRYW1fq7LRFd9Q@mail.gmail.com>
 <CAMGcDxdn8SQMuGWZ-hDVnkXv_eAbPzWRmFNU36a63b7zPeUhiw@mail.gmail.com>
 <CAMGcDxdW0LMpjxwnC6_YT2UoPgY9onXAEAbJwYDeux2mSuekOw@mail.gmail.com>
 <0c7e9fb9-01f7-32! 84-da9e-22b6dde679d4@2ndquadrant.com>
 <CA+TgmoZfuGSRBv=LDK2mk4Q1MB9wjMuL3AQfXAax2orDWerFUw@mail.gmail.com>
 <15404.1531761677@sss.pgh.pa.us>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <ad722d03-4468-b5f7-8c59-8ac79ff3c2d0@2ndquadrant.com>
Date: Mon, 16 Jul 2018 19:38:28 +0200
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On 07/16/2018 07:21 PM, Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> I agree.  As a general statement, I think the idea of trying to
>> prevent transactions from aborting is really scary.  It's almost an
>> axiom of the system that we're always allowed to abort, and I think
>> there could be a lot of unintended and difficult-to-fix consequences
>> of undermining that guarantee.  I think it will be very difficult to
>> create a sound system for delaying transactions, and I doubt very much
>> that the proposed system is sound.
> 
> Ugh, is this patch really dependent on such a thing?
> 

Unfortunately it does :-( Without it the decoding (or output plugins) 
may see catalogs broken in various ways - the catalog records may get 
vacuumed, HOT chains are broken, ... There were attempts to change that 
part, but that seems an order of magnitude more invasive than this.

> TBH, I think the odds of making that work are indistinguishable from zero;
> and even if you managed to commit something that did work at the instant
> you committed it, the odds that it would stay working in the face of later
> system changes are exactly zero.  I would reject this idea out of hand.
> 

Why? How is this significantly different from other patches touching 
ProcArray and related bits?


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services