Which column in an explain statement shows the actual index that was used to retrieve rows?

In an EXPLAIN statement, the column that indicates the actual index used to retrieve rows is identified as "key." This designation provides critical information to developers and database administrators about the efficiency of query execution. When a query is executed, the database engine evaluates various index options to determine the most effective way to access the required data. The "key" column reveals the specific index that was utilized during this process, allowing for an assessment of indexing strategies and potential performance optimization.

Understanding the index used is essential for diagnosing performance issues, as it directly impacts the query execution time and the system's overall efficiency. By leveraging this insight, developers can make informed decisions about indexing strategies, such as creating additional indexes, modifying existing ones, or refining the query itself to enhance performance.

The other columns in the EXPLAIN output provide different types of information, such as predictive index options and join methods, but none explicitly indicate which index was applied during the query execution as the "key" column does.