Re: MVCC snapshot timing

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(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +