To: Peter Eisentraut <peter_e@gmx.net>
cc: pgsql-advocacy@postgresql.org, josh@agliodbs.com,
 pgsql-docs@postgresql.org
Subject: Re: [DOCS] Avoiding upgrade backlash 
In-reply-to: <200711131045.04878.peter_e@gmx.net> 
References: <200711121442.31233.josh@agliodbs.com>
	<200711131045.04878.peter_e@gmx.net>
Comments: In-reply-to Peter Eisentraut <peter_e@gmx.net>
	message dated "Tue, 13 Nov 2007 10:45:04 +0100"
Date: Tue, 13 Nov 2007 12:11:43 -0500
Message-ID: <18129.1194973903@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Peter Eisentraut <peter_e@gmx.net> writes:
> Am Montag, 12. November 2007 schrieb Josh Berkus:
>> 3) If Robert gets his type-cast backport package together, the location of
>> that.

> Well, if you want to undo the changes, you don't need a backport
> package; you can just change the cast's definition.

It's actually not going to be that easy, because most of those casts
aren't even in pg_cast anymore: they have been subsumed into the
CoerceViaIO mechanism.  You'd need to resurrect the individual cast
functions before you could put entries back, too.

Another little problem is that you're likely to break as much stuff as
you fix.  An example in CVS HEAD:

regression=# select 42 || 'foo';
 ?column? 
----------
 42foo
(1 row)

regression=# select 42 like 'foo';
ERROR:  operator does not exist: integer ~~ unknown
LINE 1: select 42 like 'foo';
                  ^
HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.

OK, let's "fix" that by making int->text implicit again:

regression=# create function inttotext(int) returns text as $$
regression$# begin return $1; end$$ language plpgsql strict immutable;
CREATE FUNCTION
regression=# create cast (int as text) with function inttotext(int)
regression-# as implicit;
CREATE CAST

Now LIKE works:

regression=# select 42 like 'foo';
 ?column? 
----------
 f
(1 row)

but || not so much:

regression=# select 42 || 'foo';
ERROR:  operator is not unique: integer || unknown
LINE 1: select 42 || 'foo';
                  ^
HINT:  Could not choose a best candidate operator. You might need to add explicit type casts.

			regards, tom lane