Message-ID: <20041006203642.19308.qmail@web50004.mail.yahoo.com>
Date: Wed, 6 Oct 2004 15:36:42 -0500 (CDT)
From: Jaime Casanova <systemguards@yahoo.com>
Subject: Table locks
To: pgsql-sql@postgresql.org
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 --- Tom Lane <tgl@sss.pgh.pa.us> escribió: 
> Andreas Seltenreich <seltenreich@gmx.de> writes:
> > Jake Stride writes:
> >> I thought of doing:
> >> SELECT max(jobno) from jobs where companyid=1;
> 
> > I think SELECT FOR UPDATE should work fine here.
> 
> Nope; he'll get something like
> 
> regression=# select max(unique1) from tenk1 for
> update;
> ERROR:  SELECT FOR UPDATE is not allowed with
> aggregate functions
> 
> His best bet is probably
> 
> 		BEGIN;
> 		LOCK TABLE jobs;
> 		SELECT max(jobno) from jobs where companyid=1;
> 		INSERT INTO jobs ...
> 		COMMIT;
> 
> This is pretty horrid from a concurrency point of
> view but I don't think
> there's any other way to meet the "no gaps"
> requirement.
> 
> You could reduce the strength of the lock a bit, for
> instance
> 		LOCK TABLE jobs IN EXCLUSIVE MODE;
> which would allow readers of the jobs table to
> proceed concurrently,
> but not writers.  If you were willing to assume that
> all inserters into
> jobs are cooperating by explicitly obtaining the
> correct lock, you
> could reduce it to
> 		LOCK TABLE jobs IN SHARE UPDATE EXCLUSIVE MODE;
> which is the lowest self-conflicting table lock
> type.  This would allow
> unrelated updates to the jobs table to proceed
> concurrently too (though
> not VACUUMs).  See
> 
>
http://www.postgresql.org/docs/7.4/static/explicit-locking.html
> 
> 			regards, tom lane
> 

Hi, 
Talking about lock tables there is a way to do a
select ... for update
and then a
update .. where current of ... 

I think it require a select for update in a cursor.

Thanx in advance,
Jaime Casanova


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