On 06/30/2017 11:32 PM, rajan wrote:
have trouble understanding xmin and xmax with update operations from two
different sessions
So, as found below, Session2 is trying to update a row which is already
updated with a different value and it's update fails with *UPDATE 0*
But from Session3, I see that xmax value is visible as Session2's txid. Why
is it like that?
Can we not put Session2's txid to xmin instead(although the update failed)?
And if we try to consider that xmax is update with Session2's txid bcoz the
update failed, then why bother updating the xmax?
Please help me understand this.
One thing to remember is that txid_current() is more then informational:
https://www.postgresql.org/docs/9.6/static/functions-info.html
txid_current() bigint get current transaction ID, assigning a new one
if the current transaction does not have one
So calling it can advance the xid manually. Some testing here showed
that what xmin or xmax is created depends on when you call txid_current
in either the original session or the concurrent sessions.
Also worth noting that an UPDATE in Postgres is a DELETE/INSERT process.
The clue is the ctid value. In Session 2 you are looking at the
original row(ctid=(0, 2) which has been marked as deleted(non-zero
xmax). In Session 3 you are looking at the new row(ctid(0, 4)).
*Session1*
testdb=# BEGIN;
BEGIN
testdb=# select ctid, xmin, xmax, * from numbers;
ctid | xmin | xmax | number
-------+--------+------+--------
(0,2) | 519107 | 0 | 14
(0,3) | 519112 | 0 | 23
(2 rows)
testdb=# select txid_current();
txid_current
--------------
519114
(1 row)
testdb=# update numbers set number=24 where number=14;
UPDATE 1
testdb=# COMMIT;
COMMIT
*Session 2*
testdb=# BEGIN;
BEGIN
testdb=# select txid_current();
txid_current
--------------
519115
(1 row)
testdb=# select ctid, xmin, xmax, * from numbers;
ctid | xmin | xmax | number
-------+--------+------+--------
(0,2) | 519107 | 0 | 14
(0,3) | 519112 | 0 | 23
(2 rows)
testdb=# select ctid, xmin, xmax, * from numbers;
ctid | xmin | xmax | number
-------+--------+--------+--------
(0,2) | 519107 | 519114 | 14
(0,3) | 519112 | 0 | 23
(2 rows)
testdb=# update numbers set number=25 where number=14;
UPDATE 0
testdb=# COMMIT;
COMMIT
*Session 3*
testdb=# select txid_current();
txid_current
--------------
519116
(1 row)
testdb=# select ctid, xmin, xmax, * from numbers;
ctid | xmin | xmax | number
-------+--------+--------+--------
(0,3) | 519112 | 0 | 23
(0,4) | 519114 | 519115 | 24
(2 rows)
-----
--
Thanks,
Rajan.
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--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
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