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Re: HOT patch - version 15

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>
Cc: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Florian Pflug <fgp(dot)phlo(dot)org(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Gregory Stark <stark(at)enterprisedb(dot)com>, PostgreSQL-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: HOT patch - version 15
Date: 2007-09-10 19:41:31
Message-ID: 200709101941.l8AJfVG20152@momjian.us (view raw or flat)
Thread:
Lists: pgsql-patches
Pavan Deolasee wrote:
> > > Pruning does generate a WAL record at the moment. Maybe you could do
> > > some kind of a quick pruning without a WAL record. Like just modify the
> > > ctid of the oldest dead tuple in the chain, or the redirecting line
> > > pointer if there is one, to point to the latest live tuple, without
> > > removing the dead tuples or the line pointers.
> >
> > I am wondering what you even mean by removing the dead tuples when
> > pruning.  I thought only defragmentation removed tuples.
> >
> >
> Pruning removes intermediate dead tuples by marking their line pointers
> ~LP_USED and redirecting the root line pointer to the first
> live/recently_dead
> tuple in the chain. OTOH defragmentation moves tuples around to repair
> fragmentation. IOW defragmentation is nothing but calling
> PageRepairFragmentation on the page.
> 
> For example, if we have a HOT chain of 5 tuples:
> 
> 1 --> 2 --> 3 --> 4 --> 5
> 
> of which, 1, 2 and 3 are DEAD, 4 is RECENTLY_DEAD and 5 is LIVE
> 
> At the end of pruning:
> 
> - line pointer of 1 is redirected to 4
> - line pointers of 2 and 3 are marked ~LP_USED
> - the offset of 4 and 5 is unchanged
> 
> At the end of defragmentation:
> 
> - 4 and 5 are moved to the end of the page to create a larger
> contiguous free space in the page.

Right.  My point is that pruning only modifies item pointers.  It does
not change the actual heap tuples.  In the quote above, how is Heikki's
"quick pruning" different from the pruning you are describing?

-- 
  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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Next:From: Heikki LinnakangasDate: 2007-09-10 19:46:31
Subject: Re: HOT patch - version 15
Previous:From: Pavan DeolaseeDate: 2007-09-10 19:29:21
Subject: Re: HOT patch - version 15

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