Re: Where's my Derby?

Tom Anderson <>
Wed, 7 Apr 2010 22:55:36 +0100
On Wed, 7 Apr 2010, Lew wrote:

Arne Vajh?j wrote:

Especially with a high update ratio then an UPDATE with a WHERE and
only an INSERT if no rows were updated could be worth considering.

Andreas Leitgeb wrote:

Would it be safe? Or could one end up with two entries for that key,
if the attempted updates happen at the same time?

Martin Gregorie wrote:

Should be OK provided you use explicit commit units rather than default
automatic commits.

Andreas Leitgeb wrote:

"Should be ..." doesn't really sound too convincing in the context of
possible concurrency problems ;-)

If the database engine supports transactions and you remember to use
them, it will be safe.

You mean, if it has them, you remember to use them, and you're happy to
live with the consequences of using the serializable isolation level,
you'll be safe.

Being able to do an 'upsert' in a single atomic operation makes it
possible to be safe much faster than having to do it with two queries.

Another trick to consider is judicious use of SELECT ... FOR UPDATE.

True. I don't think it's applicable here, though.


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