Access Methodologies:

The performance of an application is always critical. That is because the productivity of an application user straightly associates to the amount of time in which the user must sit idle although the application tries to complete work. With the database applications, performance depends widely on how fast an application can access table data. Classically, disk I/O is the primary performance determining factor for table access-the fewer disks I/O that's necessary to access table data and the better the dependent applications will perform. In common, it's best to try and minimize the amount of disk access in which applications must perform when working with database tables.

The judicious use of table indexes is the principal method to decrease disk I/O and improve the performance of table access. As like an index in a book, an index of a table column (a group of columns) permits Oracle to quickly search specific table records. Whenever an application queries a table and uses an indexed column in its selection criteria. An Oracle automatically uses the index to quickly search the target rows with minimal disk I/O, without an index, the Oracle has to read the entire table from disk to locate rows which match a selection criterion.

The presence of an index for a table is whole optional and transparent to users and developers of database applications. For instance, Applications can access table data with or without related indexes.

Whenever an index is present will help the performance of an application request, the Oracle automatically uses the index; or else, Oracle ignores the index.

The Oracle automatically updates an index to keep it in synch with its table

While indexes can dramatically improve the performance of application request, it's unwise to index every column in a table. An Indexes are meaningful only for the key columns in which application requests specifically use to search rows of interest. In addition, index maintenance generates overhead-unnecessary indexes can really slow down your system rather which n improve its performance.

Oracle8 supports various different kinds of indexes to satisfy several types of application needs. The following sections explain more about the several types of indexes in which you can create for a table's columns.

When an index is created, An Oracle fetches and sorts the columns to be indexed and stores the ROWID with index value for each row. An Oracle loads the index from the bottom up. They are physically and logically independent of the data related with the tables.