Content-Type: text/plain;
	charset=utf-8
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.120.41.1.8\))
Subject: Re: Custom ordering operator for type xid
From: Rui DeSousa <rui.desousa@icloud.com>
In-Reply-To: <3179495.1718636638@sss.pgh.pa.us>
Date: Mon, 17 Jun 2024 11:33:17 -0400
Cc: Alexander Lipatov <lipatov@mindbox.cloud>,
 pgsql-admin@postgresql.org
Content-Transfer-Encoding: quoted-printable
Message-Id: <8A9CD524-7C29-4665-BCFB-10C4B8A8CD94@icloud.com>
References: <2EA9BF93-EE2B-4B84-B21D-5EA86FA63913@mindbox.cloud>
 <3179495.1718636638@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/8A9CD524-7C29-4665-BCFB-10C4B8A8CD94%40icloud.com>
Precedence: bulk


> On Jun 17, 2024, at 11:03 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>=20
> Alexander Lipatov <lipatov@mindbox.cloud> writes:
>> **Question**: Is it safe to create custom ordering operators for the =
`xid` type and a default operator class with these operators?
>=20
> I wouldn't do it, mainly because the semantics of what you've written
> have nothing to do with the actual behavior of xids.  (The real
> comparison behavior is "circular", which can't be modeled as a total
> order, which is why there's not a built-in opclass already.)
>=20
> What is that ORM doing with XIDs anyway, and is there a good reason
> not to run away screaming from such an ill-thought-out product?
> I don't believe for a minute that this is going to be the only
> semantic issue you'll run into with an ORM that thinks it knows
> how XIDs behave despite a clear lack of even the most minimal
> investigation into the question.
>=20
> 			regards, tom lane
>=20
>=20


Assuming they are using it for opportunist locking.  I have use this =
method before but not with a system column.

i.e.=20

1. Application fetches the record with an xmin of 55, no need to =
maintain an open transaction:  select xmin, * from table where id =3D 8; =
=20
2. Application edits record
3. Application saves record:  update table set col1 =3D =E2=80=98x=E2=80=99=
 where id =3D 8 and xmin =3D 55;
4. If the record was updated by another session then xmin would be =
different, the save would fail by updating zero records, and user would =
have to reedit the record.

Not a fan of ORMs myself but I think opportunist locking has its place.