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Re: Thoughts on maintaining 7.3

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Alvaro Herrera <alvherre(at)dcc(dot)uchile(dot)cl>,"Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: Thoughts on maintaining 7.3
Date: 2003-10-03 21:37:52
Message-ID: 200310032137.h93Lbqc06748@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:
> Alvaro Herrera <alvherre(at)dcc(dot)uchile(dot)cl> writes:
> > I think what Tom is concerned about is that this hasn't been tested
> > enough with big datasets.  Also there a little loss of index pages but
> > it's much less (orders of magnitude, I think) than what was before.
> > This is because the index won't shrink "vertically".
> 
> The fact that we won't remove levels shouldn't be meaningful at all ---
> I mean, if the index was once big enough to require a dozen btree
> levels, and you delete everything, are you going to be upset that it
> drops to 13 pages rather than 2?  I doubt it.
> 
> The reason I'm waffling about whether the problem is completely fixed or
> not is that the existing code will only remove-and-recycle completely
> empty btree pages.  As long as you have one key left on a page it will
> stay there.  So you could end up with ridiculously low percentage-filled
> situations.  This could be fixed by collapsing together adjacent
> more-than-half-empty pages, but we ran into a lot of problems trying to
> do that in a concurrent fashion.  So I'm waiting to find out if real
> usage patterns have a significant issue with this or not.

Though the new code will put empty index pages into the free-space map,
will it also shrink the index file to remove those pages?  For example,
if I have 200M rows in a table, and I delete all of them except 100,
does the index shrink, or the pages just become available for reuse. 
With VACUUM FULL, we have a way to shrink the heap.  Do we shrink the
index?

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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