BUG #2821: xid cannot be casted to a different type

The following bug has been logged online:

Bug reference: 2821
Logged by: Edwin Groothuis
Email address: mavetju(at)gmail(dot)com
PostgreSQL version: 8.0 / 8.1. / 8.
Operating system: FreeBSD
Description: xid cannot be casted to a different type
Details:

This statement is part of a longer one, which we used on 8.0
to determine the status of the locks on the database:

select relation,transaction::bigint,count(*) as waiting from
pg_locks where not granted group by relation,transaction;

This worked fine on 8.0.x. Now I'm preparing to move to 8.1
and/or 8.2, but that above statement now gives me:

ERROR: could not identify an ordering operator for type xid
HINT: Use an explicit ordering operator or modify the query.

According to a discussion on IRC with neilc and davidfetter:

<neilc> davidfetter, Mavvie: we need an ordering operator to do GROUP
BY
<neilc> for some reason that's not clear to me atm
<neilc> perhaps because originally we only implemented grouping / aggs
via sorting?
<davidfetter> neilc, makes sense, in a way
<davidfetter> yeah
<neilc> anyway, so the problem is just there aren't ordering operators
for the xid type
<Mavvie> neilc: I've tried to cast the transaction field (which is the
xid) to a text, bigint or something else but it keeps coming back as "cannot
cast type xid to ..."
<neilc> there probably should be, like we added for tid i think in 8.2
<davidfetter> would this be an initdb-forcing thing? :f
<neilc> well, you could package the changes as sql
<davidfetter> cool :)
<neilc> i wouldn't personally bother backporting it tho, it's not a bug
per se
<davidfetter> hrm. i'd say anything that causes you not to be able to do
"expected" operations like aggregation is a bug