On Fri, Mar 03/22/13, 2013 at 06:16:11AM -0700, 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: > > > > CREATE TABLE demo (id serial, value text); > > > > -- generate a few pages of dummy data > > INSERT INTO demo (value) > > SELECT md5(s.a::text) > > FROM generate_series(1, 1000) AS s(a); > > > > -- 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. >
I'm having trouble understanding why it is necessary to generate a new tuple even when nothing has changed. It seems that the OP understands that MVCC is at work, but is questioning why this exact behavior occurs. I too have the same question. Perhaps you could provide an example where an replacing the tuple would be required in the presence of multiple transactions? -Ryan Kelly -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
