An index is a system used to make finding information easier.
An index is a list of words or phrases and associated pointers to where useful material relating to that heading can be found in a document. In a traditional back of the book index the headings will include names of people, concepts selected by a person as being relevant and of interest to a possible reader of the book.
The pointers are typically page numbers, paragraph numbers or section numbers. In a library catalog the words are authors, titles, subject headings, and the pointers are call numbers. Internet search engines, such as Google, and full text searching help provide access to information but are not as selective as an index, as they provide non-relevant links, and may miss relevant information if it is not phrased in exactly the way they expect.
The indexer reads through the text, identifying indexable concepts. The indexer creates index headings, to represent those concepts, which are phrased such that they can be found when in alphabetical order. These headings and their associated locators are entered into specialist indexing software which handles the formatting of the index and facilitates the editing phase.
The index is then edited to impose consistency throughout the index.
Indexers must analyze the text to enable presentation of concepts and ideas in the index that may not be named within the text. The index is intended to help the reader, researcher, rather than the author, find information, so the professional indexer must act as a liaison between the text and the its ultimate user.
Indexing is often done by freelancers hired by authors, publishers or book packagers. Some publishers and database companies employ indexers. There are several dedicated, indexing software programs available to assist with the special sorting and copying needs involved in index preparation.
A database index is a data structure that improves the speed of operations on a database table. Indexes can be created using one or more columns of a database table, providing the basis for both rapid random look ups and efficient access of ordered records. The disk space required to store the index is typically less than that required by the table, yielding the possibility to store indexes in memory for a table whose data is too large to store in memory.
In a relational database, an index is a copy of one part of a table. Some databases extend the power of indexing by allowing indexes to be created on functions or expressions. An index could be created on upper, which would only store the upper case versions of the last name field in the index.
Another option sometimes supported is the use of filtered indexes, 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.
Indexes may be defined as unique or non unique.
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