On Tue, Jun 14, 2011 at 3:30 AM, AI Rumman <rummandba@gmail.com> wrote:
> Hi,
>
> I got duplicate key violate error in the db log for the following query:
> INSERT INTO tab1 ( SELECT '1611576', '1187865' WHERE NOT EXISTS (
> SELECT 1 FROM tab1 WHERE id='1611576' AND id2='1187865' ) )
>
> The error occured during production time.
> But when I manually executed the query, it inserted one row with success and
> next time it inserted 0 rows.
Unfortunately the operation above is not atomic. This is a classic
concurrency problem that everyone has to deal with -- there is no way
at present to rely on a simple row level lock to prevent concurrent
inserts to the same key. You have a few of ways to deal with this:
*) retry the statement (personally not a big fan of this method)
*) lock the table (lousy concurrency)
*) advisory lock might work, if you must have concurrency and your key
is an integer. be careful, and do not overuse the technique.
merlin