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Oracle uses special structures called undo records to help provide automatic statement-level read consistency. All data for a single query will come from a single point in time. Only data committed when a query begins will be seen by the query any changes made by other transactions after a query begins won t be seen by the query once it begins executing. If a transaction is modifying data, Oracle will write a before-image of the table data to its undo records. For example, if you update the salary of an employee from 10,500 to 11,000, the undo record will store the old salary value of 10,500. When a query begins execution, Oracle will determine the current system change number (SCN), which identifies the order in which transactions occurred in the database. When data blocks are read for this query, Oracle will only use data blocks with the SCN that it determined for this query. When it encounters data blocks for the query with a more recent SCN, Oracle will automatically go to the undo segments and reconstruct the data from the undo records stored there. Any changes made by other transactions during the query s execution will have more recent SCNs, and are disregarded, guaranteeing that only consistent data is returned for the query at hand.

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Tablespaces perform a number of key functions in an Oracle database, but the concept of a tablespace is not common to all relational databases. For instance, the Microsoft SQL Server database doesn t use this concept at all. Here s a brief list of the benefits of using tablespaces: Tablespaces make it easier to allocate space quotas to various users in the database. Tablespaces enable you to perform partial backups and recoveries based on the tablespace as a unit. Because a large object like a data warehouse partitioned table can be spread over several tablespaces, you can increase performance by spanning the tablespace over several disks and controllers. You can take a tablespace offline without having to bring down the entire database. Tablespaces are an easy way to allocate database space. You can import or export specific application data by using the import and export utilities at the tablespace level.

The test for percentage CPU usage is quite simple. The value to be compared to the thresholds is obtained directly from the ps output. There is no need for further calculation as was needed in the code for the cputime and etime monitors.

Tablespaces are now used mainly to separate related groups of tables and indexes. This may be important for you if you need to transport tablespaces across different databases and platforms using the Oracle Data Pump utility, or if you use different database block sizes for different tablespaces. If you don t think you ll be performing these administrative tasks using tablespaces, you can conceivably use just a couple of tablespaces to store all the data in your database.

Furthermore, a process can customize the CLR so that assemblies can be found in and loaded from custom locations. For example, SQL Server 2005 customizes assembly loading. Assemblies implementing managed stored procedures for SQL Server are stored in databases instead of files. As a consequence, you cannot use mixed-code assemblies to implement SQL Server stored procedures only assemblies compiled with /clr:pure or /clr:safe can be used for that purpose. To store a .NET assembly in a SQL Server database, you have to use the SQL DDL command CREATE ASSEMBLY. Once an assembly is installed in the database, static functions of a managed class can be exposed as stored procedures via the CREATE PROCEDURE command. However, for assemblies created with /clr:pure, some additional steps are necessary. Read the accompanying sidebar for the necessary instructions. Mixed-code assemblies cannot be loaded from non-file locations because they must be loaded by the operating system s loader for DLLs and EXEs. This is a requirement for mixedcode assemblies because the OS loader has to prepare the native code in a mixed-code assembly for execution. This preparation implies loading imported DLLs and resolving imported functions from the dependent DLLs. Without these steps, the native code parts of a mixed-code DLL will fail to call another DLL.

Each tablespace uses the default block size for the database, unless you create a tablespace with a different nonstandard block size As you ve already seen, Oracle lets you have multiple block sizes in addition to the default block size Because tablespaces ultimately consist of Oracle data blocks, this means that you can have tablespaces with different Oracle block sizes in the same database This is a great feature, and it gives you the opportunity to pick the right block size for a tablespace based on the data structure of the tables it contains The customization of the block size for a tablespace provides several benefits: Optimal disk I/O: Remember that the Oracle server has to read the table data from mechanical disks into the buffer cache area for processing.