Message-ID: <44924F80.7060500@archonet.com>
Date: Fri, 16 Jun 2006 07:28:16 +0100
From: Richard Huxton <dev@archonet.com>
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To: sathish kumar shanmugavelu <sathishkumar.shanmugavelu@gmail.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: concurrency problem
References: <ee0ad3a30606152114g122d0021ja7c603a8f36ae62c@mail.gmail.com>
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sathish kumar shanmugavelu wrote:
>             INSERT INTO rcp_patient_visit_monitor (
>                   entry_no, patient_id, visit_date, is_newpatient,
> visit_type, is_medical,
>                   is_review, is_labtest, is_scan, is_scopy, is_xray,
> weight, height)
>                   VALUES ((SELECT coalesce(max(entry_no)+1, 1) FROM
> rcp_patient_visit_monitor),?,current_timestamp,?,?,?,?,?,?,?,?,?,?)
> 
>    The point to note here is the select statement which gets the max
> entry_no and adds one to it and save the new value. entry_no is the primary
> key of the above table.
> 
>   Now i run the same program (different instance) from two systems, save
> the form simultaneously, only one entry is saved, in the other system the
> error says - duplicate key violates.

BEGIN;
LOCK TABLE ...
INSERT ...
COMMIT;

You'll need to handle possible errors where one client fails to get a 
lock and times out. It won't happen often, but you do need to consider 
the option.

-- 
   Richard Huxton
   Archonet Ltd