A database index is a data structure that improves the speed of operations in a table. Indices can be created using one or more columns, providing the basis for both rapid random lookups and efficient ordering of access to records. The disk space required to store the index is typically less than the storage of the table (since indices usually contain only the key-fields according to which the table is to be arranged, and excludes all the other details in the table), yielding the possibility to store indices into memory from tables that would not fit into it. In a relational databasean index is a copy of part of a table. Some databases extend the power of indexing by allowing indices to be created on functions or expressions. For example, an index could be created on upper(last_name), which would only store the uppercase versions of the last_name field in the index. Another option sometimes supported is the use of "filtered" indices, where index entries are created only for those records that satisfy some conditional expression. A further aspect of flexibility is to permit indexing on user-defined functions, as well as expressions formed from an assortment of built-in functions.

Presentation (UI), Business (logic and underlying code) and Data (from storage or other sources).

A thread is a the entity within a process that Windows schedules for execution. A thread has. The contents of a set of CPU registers representing the state of the processor. 2 stacks, 1 for the thread to use in kernel mode, and 1 for user mode. Private storage called Thread Local Storage for use by subsystems, run-time libraries, and DLLs and;. A thread ID.

There is no restriction in place except that any storage consumed beyond that provided by the base
offering may have an additional monthly charge associated with them.

System R was designed and developed over a period of 1974-79 at IBM San Jose Research Center. It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with performance at least comparable to that of existing system.

Its two subsystems are

Ø Research Storage

Ø System Relational Data System.

Data independence means that "the application is independent of the storage structure and access strategy of data". In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.

Two types of Data Independence:

Ø Physical Data Independence: Modification in physical level should not affect the logical level.

Ø Logical Data Independence: Modification in logical level should affect the view level.

NOTE: Logical Data Independence is more difficult to achieve