Types Of Indexes
One of the words used during the file organisation is the term index. In this part, let us describe this term in more detail.
We find the index of keywords at the end of every book. Notice that this index is a sorted list of page numbers (address) and keywords (index values) where the keyword can be found. In databases also an index is defined in a same way, as the pair.
The basic benefit of having sorted index pages at the end of the book is that we can situate a desired keyword in the book. We could have used the topic and sub-topic listed in the table of contents, but it is not essential that the given keyword can be found there; also they are not in any sorted series. If a keyword is not even searched in the table of contents then we require to search each of the pages to find the necessary keyword, which is going to be very cumbersome. Therefore, an index at the back of the book helps in locating the needed keyword references very simply in the book.
The same is true for the databases that have very big number of records. A database index permits fast search on the index value in database records. It will be complex to locate an attribute value in a huge database, if index on that value is not given. In such a case the value is to be searched record-by-record in the whole database which is time consuming and cumbersome. It is essential to note here that for a large database the whole records cannot be contain in the main memory at a time, therefore, data needs to be transferred from the secondary storage device which is more time consuming. Therefore, without an index it may be complex to search a database.
An index has a pair consisting of index value and a list of pointers to disk block for the records that have the similar index value. An index has such information for each stored value of index attribute. An index file is extremely small compared to a data file that stores a relation. Also index entries are ordered; so that an index can be seek using an efficient search method like as binary search. In case an index file is extremely large, we can make a multi-level index, that is index on index.
There are many parts of indexes those are categorised as:
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Primary index
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Single level index
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Spare index
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Secondary index
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Multi-level index
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Dense index
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Clustering index
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A primary index is defined on the attributes in the order of which the file is kept. This field is known as the ordering field. A primary index can be on the primary key of a file. If an index is on attributes other than candidate key fields then various records may be related to one ordering field value. This is known as clustering index. It is to be noted that there can be only single physical ordering field. Therefore, a file can have either the clustering index or primary index, not both. Secondary indexes are definite on the non-ordering fields. Therefore there can be various secondary indexes in a file, but only clustering index or one primary.