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Oracle will keep track of the order in which the sessions requested the lock When the session with the lock releases it the next session will be granted it, and so on This is known as the enqueue mechanism If you do not want a session to queue up if it cannot get a lock, the only way to avoid this is to use the WAIT or NOWAIT clauses of the SELECTFOR UPDATE command A normal SELECT will always succeed, because SELECT does not require any locks but a DML statement will hang The SELECTFOR UPDATE command will select rows and lock them in exclusive mode If any of the rows are locked already, the SELECTFOR UPDATE statement will be queued and the session will hang until the locks are released, just as a DML statement would To avoid sessions hanging, use either SELECTFOR UPDATE NOWAIT or SELECT FOR UPDATE WAIT <n>, where <n> is a number of seconds Having obtained the locks with either of the SELECTFOR UPDATE options, you can then issue the DML commands with no possibility of the session hanging It is possible to append the keywords SKIP LOCKED to a SELECT FOR UPDATE statement, which will return and lock only rows that are not already locked by another session This command existed with earlier releases but is only supported from release 11g
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When a session requests a lock on a row or object and cannot get it because another session has an exclusive lock on the row or object, it will hang This is lock contention, and it can cause the database performance to deteriorate appallingly as all the sessions queue up waiting for locks Some lock contention may be inevitable, as a result of normal activity: the nature of the application may be such that different users will require access to the same data But in many cases, lock contention is caused by program and system design The Oracle database provides utilities for detecting lock contention, and it is also possible to solve the problem in an emergency A special case of lock contention is the deadlock, which is always resolved automatically by the database itself Lock contention is a common reason for an application which performs well under test to grind to halt when it goes production, and the number of concurrent users increasesThis is not the DBA s fault, but be prepared to detect such problems
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It may be that the nature of the business is such that users do require write access to the same rows at the same time If this is a limiting factor in performance of the system, the only solution is business process reengineering, to develop a more efficient business model But although some locking is a necessary part of business data processing, there are some faults in application design that can exacerbate the problem Long-running transactions will cause problems An obvious case is where a user updates a row and then does not commit the change Perhaps he/she even goes off to lunch, leaving the transaction unfinished You cannot stop this happening if users have access to the database with tools such as SQL*Plus, but it should never occur with well-written software The application should take care that a lock is only imposed just before an update occurs, and released (with a COMMIT or ROLLBACK) immediately afterward Poorly written batch processes can also cause problems, if they are coded as long transactions Consider the case of an accounting suite nominal ledger: it is a logical impossibility in accountancy terms for the ledger to be partly in one period and partly in another, so the end-of-month rollover to the next period is one business transaction This transaction may involve updating millions of rows in thousands of tables, and take hours to complete If the rollover routine is coded as one transaction with a COMMIT at the end, millions of rows will be locked for hours but in accountancy terms, this is what should happen Good program design would avoid the problem by updating the rows in groups, with regular commits but the programmers will also have to take care of simulating read consistency across transactions and handle the situation where the process fails part way through If it were one transaction, this wouldn t be a problem: the database would roll it back If it is many small transactions, they will have to manage a ledger that is half in one period and half in another These considerations should not be a problem: your programmers should bear in mind that long transactions impact on the usability of the system and design their systems accordingly Third-party user process products may impose excessively high locking levels For example, there are some application development tools that always do a SELECT FOR UPDATE to avoid the necessity of requerying the data and checking for changes Some other products cannot do row-level locking: if a user wants to update one row, the tool locks a group of rows perhaps dozens or even hundreds If your application software is written with tools such as these, the Oracle database will simply do what it is told to do: it will impose numerous locks that are unnecessary in business terms If you suspect that the software is applying more locks than necessary, investigate whether it has configuration options to change this behavior
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