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 -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
