How Can Covered Indexes Speed Up Database Queries?

Unveiling the Secrets of Covered Indexes in Databases

In the vast realm of database optimization techniques, the term "covered index" often arises. Let's delve into its meaning and explore its significance.

A covered index is an index that contains all (and potentially more) needed columns for a given query. Unlike traditional indexes, which simply enable faster row identification based on search criteria, covered indexes also store the actual data required by the query.

Consider this scenario: a query like "SELECT * FROM tablename WHERE criteria" typically relies on indexes to locate rows matching the criteria. However, it then proceeds to the actual table to retrieve the data for these rows.

With a covered index spanning columns column1, column2, and column3, the same query ("SELECT column1, column2 FROM tablename WHERE criteria") can leverage the index for both row identification and data retrieval. This is because the index already includes the necessary columns. By eliminating the need to access the table, the query execution becomes significantly faster.

The benefits of covered indexes extend to scenarios where a query frequently uses specific columns for row filtering and then retrieves additional columns. By including these additional columns in the covered index, the query processor can retrieve all required data directly from the index, further enhancing performance.

To illustrate, let's say a query often uses columns A and B for filtering and subsequently retrieves columns C and D. A well-designed covered index could include columns A, B, C, and D, enabling the query to avoid accessing the table and fetching data exceptionally efficiently.

To delve deeper into the practicality of covered indexes, you can refer to this informative article: Index Covering Boosts SQL Server Query Performance.

The above is the detailed content of How Can Covered Indexes Speed Up Database Queries?. For more information, please follow other related articles on the PHP Chinese website!