What are exclusive locwaits?
• If a process wants to lock a database object exclusively, but this lock is prevented by a another database transaction (a ‘blocking lock’), then the system blocks the lock request until the current database transaction releases the object again. Generally, the locked object is a table entry. It may also be an entire table, or a stored procedure for example.
Effect to the system?
• If a blocked database transaction itself has a blocking lock, this sets off a chain of lock situations. Such blocking lock situations reduce the maximum degree of parallelism and therefore generally impair the system performance as well, since there is a delay in executing database transactions and the application therefore waits.
• Since the queue time belongs to the database time, it is possible for only a few locks (which last a long time) to greatly extend the average execution time of an SQL statement. You must consider this when you run an SQL statement analysis. The average database time may also be extended correspondingly for a certain transaction in the workload monitor (Transaction ST03).
• If the locked transaction is executed on the database by an SAP work process, then this work process is also blocked and is unavailable for processing by other SAP programs or transactions. If you execute many transactions that all need to lock the same database object, which must all therefore wait, a relatively small number of users may occupy all the available dialog work processes in the SAP system. This affects all of the SAP system users quite suddenly. The user may believe that the system has hung or crashed while it is waiting for a work process to be unlocked.
