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Re: Weird locking situation

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au>
Cc: Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Weird locking situation
Date: 2003-10-02 14:30:58
Message-ID: 26968.1065105058@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au> writes:
> What is going on here?  Surely getting a FOR UPDATE row lock should 
> prevent another process getting an update lock?

I could not duplicate your results.  I did

regression=# create table tab(id int , blah int);
CREATE TABLE
regression=# insert into tab values(1,1);
INSERT 320558 1
regression=# insert into tab values(1,2);
INSERT 320559 1
regression=# insert into tab values(2,3);
INSERT 320560 1
regression=# BEGIN;
BEGIN
regression=# SELECT * FROM tab WHERE id=1 FOR UPDATE;
 id | blah
----+------
  1 |    1
  1 |    2
(2 rows)

<< in another window >>

regression=# UPDATE tab SET blah=1 WHERE id=1;
[waits]

<< back to first window >>

regression=# UPDATE tab SET blah=1 WHERE id=1;
UPDATE 2
regression=# end;
COMMIT

<< second window now reports >>

UPDATE 2
regression=#

The behavior you describe would certainly be a bug, but you'll have to
show a reproducible example to convince me it wasn't pilot error.  One
idea that springs to mind is that maybe additional rows with id=1 were
inserted (by some other transaction) between the SELECT FOR UPDATE and
the UPDATE?

			regards, tom lane

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Subject: Re: Weird locking situation
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Subject: Re: minor view creation weirdness

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