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From: Petr Jelinek <petr.jelinek@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Nikhil Sontakke <nikhils@2ndquadrant.com>
Cc: Craig Ringer <craig@2ndquadrant.com>
Cc: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: Sokolov Yura <y.sokolov@postgrespro.ru>
Cc: Stas Kelvich <s.kelvich@postgrespro.ru>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Cc: Robert Haas <robertmhaas@gmail.com>
Subject: Re: [HACKERS] logical decoding of two-phase transactions
Date: Fri, 30 Mar 2018 00:41:56 +0200
Message-ID: <87d3e688-89fa-bd76-8841-4fc7d16bca77@2ndquadrant.com> (raw)
In-Reply-To: <ff89e6f0-baae-fd4f-6565-13e7c2f23b90@2ndquadrant.com>
References: <CAMGcDxc9p7envO8t+29j=NdQXHoba6P1btfU0d3Xeiz7zT-Mvw@mail.gmail.com>
	<20180212162040.rsnlnysmf7n4yo7i@alap3.anarazel.de>
	<CAMGcDxeViP+R-OL7QhzUV9eKCVjURobuY1Zijik4Ay_Ddwo4Cg@mail.gmail.com>
	<20180302005343.zv6nsqimcujzjrcd@alap3.anarazel.de>
	<CAMsr+YEb-f1oG+1wGji_sgswRsVQYtVXuTbVH=nnY6thMfiCqA@mail.gmail.com>
	<CAMGcDxcD2fWqUwiPWGL_d-ie-CoGTVFAsgQvQ91H93q2Jc-mhA@mail.gmail.com>
	<CAMGcDxf9HHmDOti4-DSQTtGo48R4OMki+BktPn2uTOtGr4XomQ@mail.gmail.com>
	<c167340c-0751-d54b-0f32-fa83fd414863@2ndquadrant.com>
	<CAMGcDxcGtRXtF1r3VqddzHba+BTm7q8wbgEsyhmOFuDckMTZFQ@mail.gmail.com>
	<dd5a9cb7-bb10-cb5c-834e-08dc3ffc057f@2ndquadrant.com>
	<20180329215815.36xxakgujx4luo4m@alap3.anarazel.de>
	<ff89e6f0-baae-fd4f-6565-13e7c2f23b90@2ndquadrant.com>

On 30/03/18 00:30, Petr Jelinek wrote:
> On 29/03/18 23:58, Andres Freund wrote:
>> On 2018-03-29 23:52:18 +0200, Tomas Vondra wrote:
>>>> I have added details about this in src/backend/storage/lmgr/README as
>>>> suggested by you.
>>>>
>>>
>>> Thanks. I think the README is a good start, but I think we also need to
>>> improve the comments, which is usually more detailed than the README.
>>> For example, it's not quite acceptable that LogicalLockTransaction and
>>> LogicalUnlockTransaction have about no comments, especially when it's
>>> meant to be public API for decoding plugins.
>>
>> FWIW, for me that's ground to not accept the feature. Burdening output
>> plugins with this will make their development painful (because they'll
>> have to adapt regularly) and correctness doubful (there's nothing
>> checking for the lock being skipped).  Another way needs to be found.
>>
> 
> I have to agree with Andres here. It's also visible in the latter
> patches. The pgoutput patch forgets to call these new APIs completely.
> The test_decoding calls them, but it does so even when it's processing
> changes for committed transaction.. I think that should be avoided as it
> means potentially doing SLRU lookup for every change. So doing it right
> is indeed not easy.

Ah turns out it actually does not need SLRU lookup in this case (I
missed the reorder buffer call), so I take that part back.

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




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