SELECT FOR UPDATE should work. Did you use it in a transaction? For example:
Session 1:
CREATE TABLE accounts (amount float8);
INSERT INTO accounts VALUES (10);
BEGIN;
SELECT * FROM accounts FOR UPDATE;
Session 2:
BEGIN;
SELECT * FROM accounts FOR UPDATE; <---- This should block
Session 1:
UPDATE accounts SET amount = 1;
END;
Session 2:
<--- This should now return '1'
END;
Hope that helps,
Mike Mascari
mascarm@mascari.com
-----Original Message-----
From: drevil@sidereal.kz [SMTP:drevil@sidereal.kz]
Sent: Friday, March 09, 2001 4:13 PM
To: pgsql-general@postgresql.org
Subject: [GENERAL] How to use locks, for DB noivces?
I've never used the locking features of Postgres before, and now I
find that I need to use them. I read through the instructions, but
I've never taken a database theory course so I don't understand all
the terms they used.
Here's what I need to do: The database keeps accounts. If one process
is accessing an account, no other process should be able to access it
at the same time. I need to do the following logical sequence of
things:
1. Lock the account
2. Check to see if the change to the account is permitted
3. Make the change
4. Unlock the account
I need this so that there isn't a race condition. Ie, if there are
$10 in the account, and one backen says "withdraw $9" and the other
also says "withdraw $9" at the same time, I need to make sure they
they don't execute at the same time, which would result in a negative
balance.
I tried to do this with SELECT FOR UPDATE but I couldn't get that to
work.
Thanks
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