Re: Read Uncommitted
От
Simon Riggs
Тема
Re: Read Uncommitted
Дата
Msg-id
CANP8+jLw2pgqwe2OPozVxO0e+O5JySF5MLnFRX8hua5kqYn3Nw@mail.gmail.com
Ответ на
Re: Read Uncommitted (Tom Lane)
Список
Дерево обсуждения
Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted Tom Lane <tgl@sss.pgh.pa.us>
Re: Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted Robert Haas <robertmhaas@gmail.com>
Re: Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted Tom Lane <tgl@sss.pgh.pa.us>
Re: Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted Andres Freund <andres@anarazel.de>
Re: Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted Andres Freund <andres@anarazel.de>
Re: Read Uncommitted Craig Ringer <craig@2ndquadrant.com>
Re: Read Uncommitted Tom Lane <tgl@sss.pgh.pa.us>
Re: Read Uncommitted Craig Ringer <craig@2ndquadrant.com>
Re: Read Uncommitted Mark Dilger <hornschnorter@gmail.com>
Re: Read Uncommitted Mark Dilger <hornschnorter@gmail.com>
Re: Read Uncommitted Andres Freund <andres@anarazel.de>
Re: Read Uncommitted Robert Haas <robertmhaas@gmail.com>
Re: Read Uncommitted Stephen Frost <sfrost@snowman.net>
Re: Read Uncommitted "Finnerty, Jim" <jfinnert@amazon.com>
Re: Read Uncommitted Tom Lane <tgl@sss.pgh.pa.us>
Re: Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted Mark Dilger <hornschnorter@gmail.com>
Read Uncommitted regression test coverage Mark Dilger <hornschnorter@gmail.com>
Re: Read Uncommitted regression test coverage Tom Lane <tgl@sss.pgh.pa.us>
Re: Read Uncommitted regression test coverage Mark Dilger <hornschnorter@gmail.com>
Re: Read Uncommitted Heikki Linnakangas <hlinnaka@iki.fi>
Re: Read Uncommitted Robert Haas <robertmhaas@gmail.com>
Re: Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted Bernd Helmle <mailings@oopsware.de>
Re: Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted David Steele <david@pgmasters.net>
Re: Read Uncommitted Konstantin Knizhnik <k.knizhnik@postgrespro.ru>
Re: Read Uncommitted Simon Riggs <simon@2ndquadrant.com>
Re: Read Uncommitted Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
On Wed, 18 Dec 2019 at 14:06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Simon Riggs <simon@2ndquadrant.com> writes:
> I present a patch to allow READ UNCOMMITTED that is simple, useful and
> efficient.
Won't this break entirely the moment you try to read a tuple containing
toasted-out-of-line values? There's no guarantee that the toast-table
entries haven't been vacuumed away.
I suspect it can also be broken by cases involving, eg, dropped columns.
There are a lot of assumptions in the system that no one will ever try
to read dead tuples.
This was my first concern when I thought about it, but I realised that by taking a snapshot and then calculating xmin normally, this problem would go away.
So this won't happen with the proposed patch.
The fact that you can construct a use-case in which it's good for
something doesn't make it safe in general :-(
I agree that safety is a concern, but I don't see any safety issues in the patch as proposed.
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