Covering, Composite, or Column Index: Which MySQL Index Should I Use?

Covering vs Composite vs Column Index in MySQL

Understanding the different types of indexes can significantly improve your MySQL query performance. In this article, we will delve into covering, composite, and column indexes, and address common questions regarding their usage.

Covering vs Column Index

A covering index includes all the columns that are retrieved by a query, excluding the columns used in the WHERE clause. It allows the query engine to retrieve the necessary data directly from the index without accessing the table data. On the other hand, a column index is created on a single column.

Composite vs Column Index

A composite index is created on multiple columns. The order of the columns in the index definition is crucial as only the leftmost part of the composite index can be used for indexing. If a composite index is defined as (col3, col4) and the query uses only col3 in the WHERE clause, only part of the composite index will be used.

Index Use in Queries

MySQL typically selects the index with the highest cardinality for a specific query. In a situation where separate indexes exist on different columns, the one with more distinct values will be preferred. It is generally not possible to use multiple indexes for the same table in a single query. However, subqueries or compound indexes can be used as workarounds.

Performance Implications

Using a covering index can significantly improve performance by eliminating the need to access the table data. However, creating too many covering indexes can lead to index bloat and performance degradation. Compound indexes are generally more efficient than column indexes when multiple columns are involved in a query.

MyISAM vs InnoDB Differences

The behavior of indexes differs slightly between MyISAM and InnoDB storage engines. In InnoDB, the primary key is implicitly included in all secondary indexes, making them effectively compound indexes. As a result, InnoDB efficiently uses covering indexes even when the primary key is not explicitly used in the WHERE clause. MyISAM, on the other hand, does not have this behavior.

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