Saturday, 21 November 2015

Secondary index

Secondary index / Secondary indexing / Non-clustering index / Non-clustered index




Secondary index



An index whose order of entries is different from the order of the data file is called as secondary index.


Another definition



A non-clustering, non-clustered, or secondary index specifies an order that is different from the file/table/relation’s sequential order.


Example


In this example, the left side table is the index table, and the right side is the Students table (Data file). The search key or the ordering key field in the index table is the Name of the students, which is neither primary key nor unique attribute of the students table. Name attribute is just an attribute in the students table. In our example, index is sorted on Name attribute, and Data file is sorted on regno attribute.

Secondary index - example


Points to know about secondary index:


  • Secondary index must be a dense index

  • Order of the search key values in the index file and the order of the actual records that are related to the index entries are in different order.

  • Secondary Indexes facilitate query-answering on attributes other than primary keys – or, more generally, on non-ordering attributes.

  • A file can have several secondary indexes.
 

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