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Re: Improvement of procArray.xmin for VACUUM

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, PostgreSQL-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Improvement of procArray.xmin for VACUUM
Date: 2007-03-27 00:50:48
Message-ID: 200703270050.l2R0omk14530@momjian.us (view raw or flat)
Thread:
Lists: pgsql-patches
Gregory Stark wrote:
> 
> I have a question about what would happen for a transaction running a command
> like COPY FROM. Is it possible it would manage to arrange to have no live
> snapshots at all? So it would have no impact on concurrent VACUUMs? What about
> something running a large pg_restore?

Interesting idea.  If the table had triggers, it would need a snapshot,
but if not, yea, that is certainly possible.

---------------------------------------------------------------------------


> 
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> 
> > On the whole though I think we should let this idea go till 8.4; 
> 
> I tend to agree but for a different reason. I think it's something that will
> open the doors for a lot of new ideas. If we put it in CVS HEAD early in 8.4 I
> think (or hope at any rate) we'll think of at least a few new things we can do
> with the new more precise information it exposes.
> 
> Just as an example, if you find you have no live snapshots can you throw out
> the combocid hash? Any tuple you find with a combocid that's been discarded
> that way must predate your current scan and therefore is deleted for you.
> 
> -- 
>   Gregory Stark
>   EnterpriseDB          http://www.enterprisedb.com
> 
> 
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-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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