Re: Where's my Derby?
On 17-04-2010 07:54, Tom Anderson wrote:
On Sun, 11 Apr 2010, Arne Vajh?j wrote:
On 07-04-2010 17:55, Tom Anderson wrote:
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.
Why?
Because you don't have to use serializable transaction isolation.
>
The UPSERT will more or less have to do the same thing. The fact that
it is one SQL command does not guarantee that it is more efficient.
Perhaps not, but the effect of the transaction isolation level on
concurrency is, AIUI, likely to be significant.
serializable transaction isolation level in this case basicly means a lock.
An UPSERT statement also needs to do a lock.
It is not obvious to me why the first lock is so much more
expensive that the second lock.
http://dev.mysql.com/doc/refman/5.0/en/replace.html
<quote>
Performance considerations:
Yeah, well, MySQL is MySQL. I wouldn't expect MySQL to behave like a
real database. But anyway:
Also, check out INSERT ... ON DUPLIATE KEY UPDATE... as an alternative
if you're willing to stick to MySQL 4.1+
</quote>
ON DUPLICATE KEY UPDATE is the standard way to do UPSERT on MySQL, and
isn't covered by this dire warning.
No. But it still has to the work.
Arne