| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Mark Lewis <mark(dot)lewis(at)mir3(dot)com> |
| Cc: | David Lang <dlang(at)invendra(dot)net>, Greg Stark <gsstark(at)mit(dot)edu>, Markus Schaber <schabi(at)logix-tt(dot)com>, pgsql-performance(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: qsort again (was Re: [PERFORM] Strange Create Index |
| Date: | 2006-02-17 19:43:06 |
| Message-ID: | 11801.1140205386@sss.pgh.pa.us |
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| Lists: | pgsql-hackers pgsql-performance |
Mark Lewis <mark(dot)lewis(at)mir3(dot)com> writes:
> I think we're actually on the same page here; you're right that the
> constraint above ( f(a)==f(b) iff a==b ) can't be extended to data types
> with more than 32 bits of value space. But the constraint I listed was
> actually:
> if a==b then f(a)==f(b)
I believe Martijn had it right: the important constraint is
f(a) > f(b) implies a > b
which implies by commutativity
f(a) < f(b) implies a < b
and these two together imply
a == b implies f(a) == f(b)
Now you can't do any sorting if you only have the equality rule, you
need the inequality rule.
regards, tom lane
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