Date: Tue, 12 Nov 2013 17:49:00 -0800 (PST)
From: David Johnston <polobo@yahoo.com>
To: pgsql-docs@postgresql.org
Message-ID: <1384307340948-5778039.post@n5.nabble.com>
In-Reply-To: <20131113013935.GB24549@momjian.us>
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Subject: Re: MVCC snapshot timing
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Bruce Momjian wrote
> Oh, OK, it sounds fine.  The user really doesn't choose what timeline to
> see --- rather, it is the current xid at the time they take their
> snapshot and other running xids that controls that.  You can control
> your transaction isolation level, but that only controls how often you
> take snapshots.

^ This kind of makes my point.

You've described perfectly the mechanics of the system but from a mostly
black-box perspective a user-decision (choosing the isolation level)
directly impacts which point-in-time (xid) is chosen for the snapshot behind
a particular SQL statement.  The fact they can only choose between two
pre-defined and relative points-in-time is a detail to explain later but the
fact is they are required to make such a choice (one is provided by default
but still one chooses - even if through ignorance - to use the default) as
an outcome of MVCC should be included - in some form - in the introduction. 
In the current, proposed, and my revisions it is indeed covered but to
various degrees of detail and low/high level focus.

David J.


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