Re: Rewritten rows on unchanged values

Adrian Klaver wrote:
> On 03/22/2013 05:32 AM, Bertrand Janin wrote:
>> I noticed how rows were re-written to a different location (new ctid) even
>> without changes to the values. This illustrate what I mean:

>> -- ctid = (0,1)
>> SELECT id, xmin, ctid, value
>> FROM demo
>> WHERE id = 1;
>>
>> UPDATE demo
>> SET value = value
>> WHERE id = 1;
>>
>> -- ctid = (8,41)
>> SELECT id, xmin, ctid, value
>> FROM demo
>> WHERE id = 1;
>>
>> I'm curious as to what would prevent keeping the row where it is and maybe
>> change xmin in place?
>
> Because Postgres uses MVCC:
>
> http://www.postgresql.org/docs/9.2/static/mvcc-intro.html
>
> So an update is a delete and an insert and you are really seeing a new row.

You could use a BEFORE UPDATE trigger that returns
NULL if all fields are unchanged, but I'm not sure if
that is what the OP is looking for.

Yours,
Laurenz Albe