An index in a database is a list of values in a table with the storage locations of rows in the table that contain each value. In a database, an index allows the database program to find data in a table without scanning the entire table.Indexes can be created on either a single column or a combination of columns in a table and are implemented in the form of B-trees. An index contains an entry with one or more columns (the search key) from each row in a table. A B-tree is sorted on the search key, and can be searched efficiently on any leading subset of the search key. For example, an index on columns A, B, C can be searched efficiently on A, on A, B, and A, B, C. The performance benefits of indexes, however, do come with a cost. Tables with indexes require more storage space in the database. Also, commands that insert, update, or delete data can take longer and require more processing time to maintain the indexes.
When you design and create indexes, you should ensure that the performance benefits outweigh the extra cost in storage space and processing resources.
