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Date: Mon, 11 Nov 2013 21:19:54 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: Re: MVCC snapshot timing
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On Mon, Nov 11, 2013 at 08:59:35PM -0500, Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > On Mon, Nov 11, 2013 at 03:39:45PM -0500, Tom Lane wrote:
> >> I'm not really seeing the point of s/transaction/session/ here.
> 
> > Well, the problem with the original wording is that we don't take a new
> > snapshot for every transaction in the default read-committed mode. 
> 
> We take at least one snapshot per transaction, in any mode.  Referring
> to sessions makes it even further away from being a useful concept.
> 
> > Would you prefer I refer to statements, e.g.:
> 
> 'Statement' might work.

OK, updated patch attached.  Is "statement" too vague here?  SQL
statement?  query?

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

  + Everyone has their own god. +

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diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
new file mode 100644
index cefd323..7d6e75b
*** a/doc/src/sgml/mvcc.sgml
--- b/doc/src/sgml/mvcc.sgml
***************
*** 41,52 ****
      for developers to manage concurrent access to data.  Internally,
      data consistency is maintained by using a multiversion
      model (Multiversion Concurrency Control, <acronym>MVCC</acronym>).
!     This means that while querying a database each transaction sees
      a snapshot of data (a <firstterm>database version</firstterm>)
      as it was some
      time ago, regardless of the current state of the underlying data.
!     This protects the transaction from viewing inconsistent data that
!     could be caused by (other) concurrent transaction updates on the same
      data rows, providing <firstterm>transaction isolation</firstterm>
      for each database session.  <acronym>MVCC</acronym>, by eschewing
      the locking methodologies of traditional database systems,
--- 41,52 ----
      for developers to manage concurrent access to data.  Internally,
      data consistency is maintained by using a multiversion
      model (Multiversion Concurrency Control, <acronym>MVCC</acronym>).
!     This means that while querying a database each statement sees
      a snapshot of data (a <firstterm>database version</firstterm>)
      as it was some
      time ago, regardless of the current state of the underlying data.
!     This prevents statements from viewing inconsistent data produced
!     by concurrent transactions performing updates on the same
      data rows, providing <firstterm>transaction isolation</firstterm>
      for each database session.  <acronym>MVCC</acronym>, by eschewing
      the locking methodologies of traditional database systems,

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