Should i use SELECT for update NOWAIT ?

Question

Hi:
Do I need to use, in my pl/sql triggers and procedures, the SELECT FOR UPDATE NOWAIT sentence, to avoid locks before using update table sentences ? Is it common to use it on stored procedures and triggers?
Thanks
Joao Oliveira

Answer 1

First, what, exactly do you mean by "avoid locks"? I was interpreting that to mean "I want to avoid creating locks in my session that might block someone else", not "I want to avoid having my SELECT wait for locks to be released-- I want it to fail immediately". If you meant the latter, then SELECT ... FOR UPDATE NOWAIT would be what you want. If you meant the former, then pessimistic locking is not what you want.
Second, what sort of Oracle Forms architecture do you have? Are you still using old-school client-server applications? Or are you using a three-tiered approach? As Tom discusses in that thread, pessimistic locking is only an option when your client application is able to maintain database state across calls (i.e. client/server systems) not when you have stateless connections (which is the norm in the three-tier model). The old client-server versions of Forms would automatically and transparently do pessimistic locking. Since you didn't mention anything about your architecture, most of us probably assumed the more common stateless client architecture (note how Tom's answers progress over the 5 years in that thread as client/server architecture became less and less common).
Third, while your question is appropriate for either the Database - General forum or the SQL and PL/SQL forum, that generally means that you are free to post it either forum, not that it should be posted in both. The vast majority of the folks that hang out in one forum hang out in the other. It's also rather frustrating to answer a post in one forum only to discover that there is another post in a different forum where someone else had already covered the same points half an hour earlier or to discover that there was additional information in another thread that might have changed your answer.
Fourth, if you are going to do pessimistic locking, that requires that you are able to maintain state across various database calls, that you are locking on the lowest possible level of granularity, and that you are able to time out sessions relatively aggressively to ensure that someone doesn't open a record, thereby locking it, go to lunch (or have their system die) and then block everyone else from working. Assuming that is the case, and that you have some reasonable way to handle the error that gets generated other than simply retrying the operation, adding NOWAIT is certainly an option. Most applications, particularly those getting written today, cannot guarantee all these things, so pessimistic locking is generally not appropriate there.
Looking at your other thread (where there is new information that would be useful in this discussion, one of the reasons that multiple threads are generally a bad idea), it seems that you have an ERP application and you are concerned about the performance of entering orders. Obviously, there shouldn't be any locking issues on the ORDER or ORDER_DETAILS tables, assuming that multiple users aren't going to be inserting the same order at the same time. The contention would almost certainly come when multiple orders are trying to update the STOCK and INVENTORY tables, since multiple orders presumably rely on the same rows in those tables. In that case, I'm not sure what adding a NOWAIT would buy you-- unless you were going to roll back the entire order because someone is updating the STOCK row for #2 pencils and your order has an item of #2 pencils, you'd have to keep retrying the operation until you were able to modify the STOCK row, which would be less efficient than just letting that update block until the row was free.
Now, you could certainly redesign the application to minimize that contention by not trying to update what I assume are aggregate tables like STOCK and INVENTORY directly as part of your OLTP processing or, at least, by minimizing the time that you're locking a row. You could, for example, make STOCK and INVENTORY materialized views rather than tables that refresh ON COMMIT, which should decrease the time that your locks are held. You could also have those tables refreshed asynchronously, which would be even more efficient but may require that you reasses your holdback requirements.
Justin