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When you use automatic PGA memory management by setting the PGA_AGGREGATE_TARGET parameter, Oracle will do its best to assign enough memory to all work areas so they work in an optimal manner, executing all memory-intensive SQL operations in the cache memory. At worst, some work areas will use the disk areas in a single-pass mode. However, if you set the PGA_AGGREGATE_TARGET parameter too low relative to the work area needs of your instance, Oracle will be forced to conduct multi-pass executions of the sort- or hash-intensive SQL operations, with disastrous results for your instance performance.

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I discuss PGA management in more detail in 22, which deals with tuning instance performance, and I show how to determine the optimal size for your PGA_AGGREGATE_TARGET initialization parameter.

So far in this chapter, you ve seen the components of the Oracle database system: the necessary files and memory allocations and how you can adjust them It s time now to look into how Oracle processes users queries and how it makes changes to data It s important to understand the mechanics of SQL transaction processing because all interaction with an Oracle database occurs either in the form of SQL queries that read data or SQL (or PL/SQL) operations that modify, insert, or delete data A transaction is a logical unit of work in an Oracle database, and consists of one or more SQL statements A transaction begins with the first executable SQL statement and terminates when you commit or roll back the transaction Committing a transaction will make your changes permanent, and rolling back the changes will, of course, undo them.

Usually, native C++ exception handling is used much more often than Win32 SEH. C++/CLI allows you not only to catch C++ exceptions in managed code, but also to mix C++ exception handling with managed exception handling. A single try block can have catch blocks for C++ exceptions as well as managed exceptions, as the following code shows: // CPlusPlusExceptions.cpp // compile with "cl /clr CPlusPlusExceptions.cpp" using namespace System; // As I will discuss later, #pargma managed is not recommended; it is only // used to show exceptions thrown across managed / unmanaged boundaries // without using two source files #pragma managed(push, off) void f() { throw 4; } #pragma managed(pop) int main() { try { f(); } catch (int i) { Console::WriteLine("int exception, value={0}", i); } catch (Exception^ ex) { Console::WriteLine(ex->GetType()->FullName); } }

Once you commit the transaction, all other users transactions that start subsequently will be able to see the changes made by your transactions When a transaction fails to execute completely (say, due to a power failure), the entire transaction must be undone Oracle will roll back any changes made by earlier SQL statements in the transaction, leaving the data in its original (pre-transaction) state The whole process is designed to maintain data consistency a transaction is an all or nothing concept The following simple example of a row being inserted outlines how Oracle processes transactions: 1 A user requests a connection to the Oracle server through a 3-tier or an n-tier web-based client using Oracle Net Services 2 Upon validating the request, the server starts a new dedicated server process for that user 3 The user executes a statement to insert a new row into a table 4.

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Oracle checks the user s privileges to make sure the user has the necessary rights to perform the insertion If the user s privilege information isn t already in the library cache, it will have to be read from disk into that cache 5 If the user has the requisite privileges, Oracle checks whether a previously executed SQL statement that s similar to the one the user just issued is already in the shared pool If there is, Oracle executes this version of the SQL; otherwise Oracle parses and executes the user s SQL statement Oracle then creates a private SQL area in the user session s PGA 6 Oracle first checks whether the necessary data is already in the data buffer cache If not, the server process reads the necessary table data from the data files on disk 7.