MySQL Index Types: Covering, Composite, and Column

In this article, we will examine the different types of indexes available in MySQL: covering, composite, and column indexes. We will explore their mechanisms, functionality, and performance implications to optimize your queries.

Query with Multiple Indexes

When executing a query with multiple indexes available, MySQL will utilize the index with the highest cardinality. Cardinality measures the uniqueness of values within an index, and the index with the most distinct values typically offers faster lookup times.

Composite Indexes and Left-Most Prefix

Unlike covering indexes, composite indexes contain multiple columns. MySQL only uses the left-most prefix of a composite index in queries. If our query only uses one field from the composite index, the prefix length can impact performance as not all index fields are leveraged.

Covering Indexes

Covering indexes encompass all selected columns in a query. By doing so, MySQL can retrieve data solely from the index without accessing the underlying table. This optimization minimizes I/O operations and improves query performance.

In InnoDB, primary keys are implicitly included in secondary indexes, effectively creating compound indexes. However, even with a covering index in queries that only retrieve indexed fields, MySQL may still prefer the primary key for faster lookup.

Influence on Storage Engines and Conclusion

The choice of index type can vary depending on the storage engine employed. While covering indexes are suitable for both InnoDB and MyISAM, composite indexes exhibit different performance characteristics. In general, selecting the appropriate index and understanding their behavior are crucial for optimizing database queries. By carefully considering these types of indexes, developers can leverage the full potential of MySQL indexing mechanisms.