From: Rui DeSousa <rui.desousa@icloud.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alexander Lipatov <lipatov@mindbox.cloud>
Cc: pgsql-admin@postgresql.org
Subject: Re: Custom ordering operator for type xid
Date: Mon, 17 Jun 2024 11:33:17 -0400
Message-ID: <8A9CD524-7C29-4665-BCFB-10C4B8A8CD94@icloud.com> (raw)
In-Reply-To: <3179495.1718636638@sss.pgh.pa.us>
References: <2EA9BF93-EE2B-4B84-B21D-5EA86FA63913@mindbox.cloud>
<3179495.1718636638@sss.pgh.pa.us>
> On Jun 17, 2024, at 11:03 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> 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?
>
> 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.)
>
> 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.
>
> regards, tom lane
>
>
Assuming they are using it for opportunist locking. I have use this method before but not with a system column.
i.e.
1. Application fetches the record with an xmin of 55, no need to maintain an open transaction: select xmin, * from table where id = 8;
2. Application edits record
3. Application saves record: update table set col1 = ‘x’ where id = 8 and xmin = 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.
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To: pgsql-admin@postgresql.org
Cc: rui.desousa@icloud.com, tgl@sss.pgh.pa.us, lipatov@mindbox.cloud
Subject: Re: Custom ordering operator for type xid
In-Reply-To: <8A9CD524-7C29-4665-BCFB-10C4B8A8CD94@icloud.com>
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