What is a Covered Index and How Does it Optimize Database Queries?

Understanding Covered Indexes

You've encountered the term "covered index" in a database context. To fully grasp its significance, let's delve deeper into what a covered index entails.

A covered index is an index that holds all the columns necessary for a specific query and potentially additional columns. This means that when a query is executed using the appropriate covered index, the database can retrieve the data directly from the index without having to access the underlying table.

Consider the following SQL query:

SELECT *
FROM tablename
WHERE criteria

Typically, indexes are employed to expedite row selection based on the "criteria," but the actual rows are retrieved from the full table. However, if the index includes columns like "column1," "column2," and "column3," the following SQL query:

SELECT column1, column2
FROM tablename
WHERE criteria

can be significantly optimized. The covered index can not only facilitate row selection but also provide the values for the desired columns, eliminating the need to access the table.

This optimization technique can be particularly advantageous when a query typically uses a small number of columns to identify rows and then adds additional columns to the result set. By including those additional columns in the covered index, the query processor can retrieve all necessary data directly from the index.

For further insights, consult this informative article: Index Covering Boosts SQL Server Query Performance.

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