Sorry for replying late... I've been firefighting some issues here ;)
So the UPDATE with ORDER BY is like a select but except that you apply your update using a given ordering.
This is useful when you're using the LIMIT at the same time, this way you could flag things given a specific order and then retrieved with a select statement what you've just flagged, ensuring that no one would flag this records at the same time as you do.
So here is how I use it.
In my case I've got a kind of message queue that needs to be processed in the ordering that the records have been inserted, and I'm basically fetching lots of records altogether to decrease network traffic and latency due to DB accesses.
So I'm doing this in a transaction to first flag and fetch events to ensure atomicity:
UPDATE FROM event_logs SET processing = $batchId WHERE processing is null ORDER BY id LIMIT 1000;
SELECT FROM event_logs WHERE processing = $batchId ORDER BY id;
Then I'm processing these events as they needs to be, but if I've got to batch reader trying to access read from the event_logs table at the same time, as I'm doing the update first, both updates will flag different records, and so it ensure that a record will be part of only one batch.
If I were to run the SELECT first, then it might be that the same event is returned many times until the processing is set to a value.
In a highly concurrent usage this happens from time to time... and when it happens it makes it very hard to debug why ;)
Hence the reason why I'm using UPDATE ... ORDER BY ... LIMIT and the fact that I can't use H2 for this as H2 is a very limited database.
The UPDATE ... ORDER BY ... LIMIT is supported by all big RDBMS vendors (Oracle, MySql, SqlServer, PostgreSQL, etc) and sqlite also supports all of this, and as sqlite supports in memory databases without any setup, it makes it a very good choice for being used in a testing framework.
I hope it answers your questions :)
Let me know if you want more details.
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