Date: Mon, 21 Nov 2005 16:00:16 -0500
From: george young <gry@ll.mit.edu>
To: pgsql-sql@postgresql.org
Subject: tid_le comparison for tuple id (ctid) values?
Message-Id: <20051121160016.282c66c2.gry@ll.mit.edu>
Reply-To: gry@ll.mit.edu
Organization: MIT Lincoln Laboratory
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[PostgreSQL 8.1.0 on i686-pc-linux-gnu]
I would like to suggest that there be a less-than (or greater-than)
operator for the 'tid' type.  

I used to use oid's for finding and distinguishing duplicate data.
Now that oid's are not included by default (and I do not quarrel with
that change), I thought I could use ctid's instead. 

Suppose I have a table steps: 
   create table steps(x text, y text, z text)
but I want there to be a primary key(x,y).  If I try to do:
   create table temp_steps(x text, y text, z text, primary key(x,y))
   insert into temp_steps select * from steps;
   drop table steps; alter table temp_steps rename to steps;

I get an error that "duplicate key violates unique constraint".  Some of the rows in steps differ only in value of z.  OK, I'll just fix the data...

I thought I could force values of x to be distinct with:
(I've done this several times in the past with oid's)

   update steps set x=x||'X' from steps s where steps.key1=s.key1 and steps.key2=s.key2 and step.ctid<s.ctid;

But this fails because there is no less-than operator (or function) on type "tid".  I tried casting the ctid to string but that fails too.  Using "not s.ctid=step.ctid" doesn't get me what I need(BTW, there's no != operator for tid either).
I don't actually care which row of a pair gets changed.  I just need a way to choose *one* in the update.  [sometimes I do a delete with similar where clause]

If there was some asymmetrical comparison defined on ctids, as there had been on oids, this would all work fine.  A cast to some type that has less-than would also work fine.  Any suggestions?

-- George
-- 
"Are the gods not just?"  "Oh no, child.
What would become of us if they were?" (CSL)