Date: Tue, 12 Nov 2013 10:14:27 -0500
From: Bruce Momjian <bruce@momjian.us>
To: David Johnston <polobo@yahoo.com>
Cc: pgsql-docs@postgresql.org
Subject: Re: MVCC snapshot timing
Message-ID: <20131112151427.GH15562@momjian.us>
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On Mon, Nov 11, 2013 at 07:25:59PM -0800, David Johnston wrote:
> This reads badly to my ears:
> 
> 
> > This means that while querying a database each SQL statement sees a
> > snapshot of data (a database version) as it was some time ago, regardless
> > of the current state of the underlying data.
> 
> How about something closer to:
> 
> 
> > This means for each SQL statement the user can specify a relative
> > point-in-time snapshot (database version) of the database against which to
> > query.  These snapshot options are 1) the most recent committed data
> > currently available database-wide - including implicit commits (see note),
> > or 2) the committed data as-of the beginning of the current transaction -
> > including any changes made in the same.
> > 
> > Note: an implicit commit occurs only within a multi-statement transaction. 
> > For the purpose of determining if data has been committed any prior
> > statements in the same transaction are deemed to have been committed when
> > viewed by later statements.
> 
> I know this is an introduction paragraph so the broad concept is being
> focused on rather than how such a user would in fact make this choice.
>  
> I don't know that the term "implicit commit" is used elsewhere, likely not,
> but in effect that is what a statement in a transaction is seeing with
> respect to prior statements in the same transaction.  Naming this behavior
> in the introduction would allow for someone less verbose descriptions to be
> used in detail sections.
> 
> The above could be better integrated into the intro but I wanted to get
> opinions on the approach first.

We just want to get across the MVCC concept in the intro --- we cover
the snapshots later in the document.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +


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