Re: Repeatable read and serializable transactions see data committed after tx start

From: Álvaro Hernández Tortosa <aht(at)8Kdata(dot)com>
To: Jim Nasby <Jim(dot)Nasby(at)BlueTreble(dot)com>, Craig Ringer <craig(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Repeatable read and serializable transactions see data committed after tx start
Date: 2014-11-06 00:04:34
Message-ID: 545ABB12.1050607@8Kdata.com
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On 05/11/14 17:46, Jim Nasby wrote:
> On 11/4/14, 6:11 PM, Álvaro Hernández Tortosa wrote:
>> Should we improve then the docs stating this more clearly? Any
>> objection to do this?
>
> If we go that route we should also mention that now() will no longer
> be doing what you probably hope it would (AFAIK it's driven by BEGIN
> and not the first snapshot).

If I understand you correctly, you mean that if we add a note to
the documentation stating that the transaction really freezes when you
do the first query, people would expect now() to be also frozen when the
first query is done, which is not what happens (it's frozen at tx
start). Then, yes, you're right, probably *both* the isolation levels
and the now() function documentation should be patched to become more
precise.

>
> Perhaps we should change how now() works, but I'm worried about what
> that might do to existing applications...

Perhaps, I also believe it might not be good for existing
applications, but it definitely has a different freeze behavior, which
seems inconsistent too.

Thanks,

Álvaro

--
Álvaro Hernández Tortosa

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