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.
Thanks,
-b
--
Adrian Klaver
adrian.klaver@xxxxxxxxx
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