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Re: MVCC and Implications for (Near) Real-Time Application

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: <pgsql-performance(at)postgresql(dot)org>, "Steve Wong" <powerpchead(at)yahoo(dot)com>
Subject: Re: MVCC and Implications for (Near) Real-Time Application
Date: 2010-10-29 17:18:20
Message-ID: 4CCABB8C0200002500036F8D@gw.wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-performance
Steve Wong <powerpchead(at)yahoo(dot)com> wrote:
 
> (1) Does the MVCC architecture introduce significant delays
> between insert by a thread and visibility by other threads (I am
> unclear about how multiple versions are "collapsed" or reconciled,
> as well as how different query threads are seeing which version)?
 
As soon as the inserting transaction commits the inserted row is
visible to new snapshots.  If you are in an explicit transaction the
commit will have occurred before the return from the COMMIT request;
otherwise it will have completed before the return from the INSERT
request.
 
You will get a new snapshot for every statement in READ COMMITTED
(or lower) transaction isolation.  You will get a new snapshot for
each database transaction in higher isolation levels.
 
-Kevin

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