2.8.2.7 Transaction Log Files
A transaction is a logical group of SQL statements that carry out a unit of work. Client server databases use log files to keep track of transactions that are applied to the database. For example, before an update is applied to a database, the database server creates an entry in the transaction log to generate a before picture of the data in a Table and then applies a transaction and creates another entry to generate an after picture of the data in that Table. This keeps track of all the operations per-formed on a database. Transaction logs can be used to recover data in case of crashes or disasters. Transaction logs are automatically maintained by SQL Server.
2.8.3 Oracle Databases
Oracle was designed to be platform independent, making it architecturally more complex than the SQL Server database. An Oracle database contains more files than a SQL Server database.
The Oracle DBMS comes in three levels: Enterprise, Standard and Personal. The Enterprise edi-tion is the most powerful and is suiTable for large installations using a large number of transactions in a multi-user environment. The Standard edition is also used by high-level multi-user installations.
It lacks some of the utilities available in the Enterprise edition. The Personal edition is used in a single-user environment for developing database applications. The database engine components are virtually the same for all three editions.
The Oracle architecture is made up of several components, including an Oracle server, Oracle instance and Oracle database. The Oracle server contains several files, processes and memory struc-tures. Some of these are used to improve the performance of the database and ensure database recovery in case of a crash. The Oracle server consists of an Oracle instance and an Oracle data-base. An Oracle instance consists of background processes and memory structures. Background processes perform input/output and monitor other Oracle processes for better performance and reli-ability. An Oracle database consists of data files that provide the actual physical storage for the data.