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Once it starts executing a SQL statement, an Oracle process doesn t always get to work on the execution of the statement without any interruptions. Often, the process has to pause or wait for some resource to be released before it can continue its execution. Thus, an active Oracle process is doing one of the following at any given time: The process is executing the SQL statement. The process is waiting for something (for example, a resource such as a database buffer or a latch). It could be waiting for an action such as a write to the buffer cache to complete. That s why the response time the total time taken by Oracle to finish work is correctly defined as follows: response time = service time + wait time When you track the total time taken by a transaction to complete, you may find that only part of that time was taken up by the Oracle server to actually do something. The rest of the time, the server may have been waiting for some resource to be freed up or waiting for a request to do something. This busy resource may be a slow log writer or a database writer process. The wait event may also be due to unavailable buffers or latches. The wait events in the V$SYSTEM_EVENT view (instance-level waits) and the V$SESSION_EVENT view (session-level waits) tell you what the wait time is due to (full table scans, high number of library cache latches, and so on). Not only do the wait events tell you what the wait time in the database instance is due to, but they also tell you a lot about bottlenecks in the network and the application.

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It s important to understand that the wait events are only the symptoms of problems, most likely within the application code. The wait events show you what s slowing down performance, but not why a certain wait event is showing up in large numbers. It s up to you to investigate the SQL code to find out the real cause of the performance problems.

Four dynamic performance views contain wait information: V$SESSION, V$SYSTEM_EVENT, V$SESSION_EVENT, and V$SESSION_WAIT. These four views list just about all the events the instance was waiting for and the duration of these waits. Understanding these wait events is essential for resolving performance issues. Let s look at the common wait events in detail in the following sections. Remember that the four views show similar information but focus on different aspects of the database, as you can see from the following summary. The wait events are most useful when you have timed statistics turned on. Otherwise, the wait events only have the number of times they occurred, not the length of time they consumed. Without timing the events, you can t tell if a wait event was indeed a contributing factor in a system slowdown.

Use the wait event views (wait interface) for examining current and recent performance issues in your instance. For comprehensive analysis of most performance problems, you need to use the ADDM, which analyzes the AWR hourly snapshots.

To handle leaking native resources, the GC supports a last-chance cleanup function. Before the GC actually reclaims an object s memory, it can call a special function on the object to inform it about its upcoming end. This function is called a finalizer. Technically spoken, a finalizer is an override of the virtual function System::Object::Finalize, but a C++/CLI programmer uses a special language construct to implement finalization logic. In addition to a destructor function ~T, a type can also have a finalization function !T. This is shown in the following code: public ref class SampleClass { ... other members ... ~SampleClass() { ... this destructor is called via IDisposable::Dispose for normal cleanup ... }

Oracle wait interface analysis has garnered quite a bit of attention in the last few years. There are entire books dedicated to Oracle waits. I discuss the important performance topic of Oracle wait analysis later in this chapter, in the section Analyzing Instance Performance. Ideally, all sessions should be on the CPU, with zero time spent waiting for resources such as I/O. However, remember that every working instance will have some kind of wait. It s unrealistic to expect to work towards a

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