SSC IMD SA 2017 – Question 186

A _________________ index is one that satisfies all the columns requested in a query without having to look up further in the clustered index.

• Clustered
• Non-clustered
• Covering
• B-tree

Detailed Explanation:

The correct answer is c) Covering index (कवरिंग).

In database performance tuning, an index is called a Covering Index when it contains all the columns needed to satisfy a particular query:

• How it Works: A covering index includes all columns referenced in the query’s SELECT list, WHERE filtering predicates, JOIN conditions, GROUP BY, and ORDER BY clauses.
• Elimination of Key / Bookmark Lookups: In a standard non-clustered index, if a query requests a column not present in the index, the database engine must execute a secondary Key Lookup or RID Lookup into the clustered index or heap to retrieve the missing data. A covering index provides all requested columns directly from its leaf pages, completely eliminating expensive lookups.
• This yields major I/O reduction and dramatically accelerates query execution speed.