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Re: pgsql-server/src/backend/utils/sort tuplesort.c

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: pgsql-committers(at)postgresql(dot)org
Subject: Re: pgsql-server/src/backend/utils/sort tuplesort.c
Date: 2004-03-18 03:06:18
Message-ID: 200403180306.i2I36Ig01265@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-committers
Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > Tom Lane wrote:
> >> During btree index build, sort equal-keyed tuples according to their
> >> TID (heap position).  This doesn't do anything to the validity of the
> >> finished index, but by pretending to qsort() that there are no really
> >> equal keys in the sort, we can avoid performance problems with qsort
> >> implementations that have trouble with large numbers of equal keys.
> >> Patch from Manfred Koizar.
> 
> > I think there is also the advantage that many equal keys will access the
> > heap in a more sequential, rather than random, order, which is the part
> > that really excited me.
> 
> But we aren't attempting to maintain that ordering after index build.
> (In fact, it was exactly that point that triggered the argument last
> time round ...)

Agreed, but we don't maintain CLUSTER either.  I see no harm in having
it start out ordered, at least.

-- 
  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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