Re: Where's my Derby?
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?
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.
http://dev.mysql.com/doc/refman/5.0/en/replace.html
<quote>
Performance considerations:
Please note that REPLACE INTO is a much slower performer than an UPDATE
statement. Keep in mind that a REPLACE INTO requires a test on the keys,
and if a matching unique key is found on any or all columns, a DELETE
FROM is executed, then an INSERT is executed. There's a lot of
management of rows involved in this, and if you're doing it frequently,
you'll hurt your performance unless you simply cannot do with any other
syntax.
The only time when I can see where you'd actually need a REPLACE INTO is
when you have multiple unique constraints on a table, and need to drop
any rows that would match any of the constraints. Then REPLACE INTO
becomes more efficient from DELETE FROM... INSERT INTO...
If you're looking at a single unique column table (Primary Key), please
use UPDATE, or INSERT. Also, check out INSERT ... ON DUPLIATE KEY
UPDATE... as an alternative if you're willing to stick to MySQL 4.1+
</quote>
does not promise blazing speed.
Arne