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Re: XMIN/xid vs UNION

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Karsten Hilbert <Karsten(dot)Hilbert(at)gmx(dot)net>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: XMIN/xid vs UNION
Date: 2004-10-29 22:27:46
Message-ID: 29765.1099088866@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-general
Karsten Hilbert <Karsten(dot)Hilbert(at)gmx(dot)net> writes:
> However, the question still holds true: Is there any
> particular reason those operators aren't found in my PG
> installation despite being listed as added since 7.2 ?

The only thing that was added in 7.2 was xid equality.

There was some talk recently of adding the other comparison operations,
but I'm not sure it's a good idea.  For many purposes you'd want such
comparison operators to use the same semantics as TransactionIdPrecedes
(ie, compare mod 2^31) but that would mean that some of the usual
arithmetic laws break down (no transitive law, no triangle inequality)
and in particular you could not sort or build a btree index with such
operators.  Which in turn means that you still wouldn't get the behavior
you wanted of having UNION or ORDER BY "just work".

			regards, tom lane

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