On Wed, Nov 16, 2011 at 07:08:27PM -0500, Tom Lane wrote:
> I wrote:
> > Simon Riggs <simon@2ndQuadrant.com> writes:
> >> We need a function called transactionid_current() so a normal user can write
>
> >> select virtualtransaction
> >> from pg_locks
> >> where transactionid = transactionid_current()
>
> >> and have it "just work".
>
> > That would solve that one specific use-case. The reason I suggested
> > txid_from_xid is that it could also be used to compare XIDs seen in
> > tuples to members of a txid_snapshot, which is not possible now.
>
> BTW, a pgsql-general question just now made me realize that
> txid_from_xid() could have another use-case too. Right now, there are
> no inequality comparisons on XIDs, which is necessary because XIDs in
> themselves don't have a total order. However, you could
>
> ORDER BY txid_from_xid(xmin)
>
> and it would work, ie, give you rows in their XID order. This could be
> useful for finding the latest-modified rows in a table, modulo the fact
> that it would be ordering by transaction start time not commit time.
Added to TODO:
Add function to allow easier transaction id comparisons
http://archives.postgresql.org/pgsql-hackers/2011-11/msg00786.php
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ It's impossible for everything to be true. +