Update TODO.detail/qsort.

doc/TODO.detail/qsort

@@ -988,3 +988,92 @@ since
 > Servus
 >  Manfred
 
+From pgsql-hackers-owner@postgresql.org Mon Dec 19 13:36:58 2005
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+Date: Mon, 19 Dec 2005 09:36:44 -0800
+Subject: Re: Re: Which qsort is used
+From: "Luke Lonergan" <llonergan@greenplum.com>
+To: "Martijn van Oosterhout" <kleptog@svana.org>,
+	"Dann Corbit" <DCorbit@connx.com>
+cc: "Tom Lane" <tgl@sss.pgh.pa.us>,
+	"Qingqing Zhou" <zhouqq@cs.toronto.edu>,
+	"Bruce Momjian" <pgman@candle.pha.pa.us>,
+	"Neil Conway" <neilc@samurai.com>,
+	pgsql-hackers@postgresql.org
+Message-ID: <BFCC2FAC.16CC0%llonergan@greenplum.com>
+Thread-Topic: [HACKERS] Re: Which qsort is used
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+Status: OR
+
+Martin,
+
+On 12/19/05 3:37 AM, "Martijn van Oosterhout" <kleptog@svana.org> wrote:
+
+> I'm not sure whether we have a conclusion here, but I do have one
+> question: is there a significant difference in the number of times the
+> comparison routines are called? Comparisons in PostgreSQL are fairly
+> expensive given the fmgr overhead and when comparing tuples it's even
+> worse.
+
+It would be interesting to note the comparison count of the different
+routines.
+
+Something that really grabbed me about the results though is that the
+relative performance of the routines dramatically shifted when the indirect
+references in the comparators went in.  The first test I did sorted an array
+of int4 - these tests that Qingqing did sorted arrays using an indirect
+pointer list, at which point the same distributions performed very
+differently.
+
+I suspect that it is the number of comparisons that caused this, and further
+that the indirection has disabled the compiler optimizations for memory
+prefetch and other things that it could normally recognize.  Given the usage
+pattern in Postgres, where sorted things are a mix of strings and intrinsic
+types, I'm not sure those optimizations could be done by one routine.
+
+I haven't verified this, but it certainly seems that the NetBSD routine is
+the overall winner for the type of use that Postgres has (sorting the using
+a pointer list).
+
+- Luke
+
+
+