MySQL Clustered vs Non-Clustered Index

The MySQL terms clustered index and non-clustered index are not used in the same way as in some other database systems like SQL Server. Instead, MySQL uses slightly different terminology and concepts for index management. The two primary types of indexes in MySQL are:

1. Primary Key Index (Implicitly Clustered): In MySQL’s InnoDB storage engine (which is the default engine as of my last knowledge update in September 2021), the primary key of a table is implicitly used as a clustered index. This means that the rows in the table are physically stored in the order of the primary key. The primary key index determines the physical layout of the data, and it ensures the uniqueness of rows.
2. Secondary Indexes (Non-Clustered): Secondary indexes are used to quickly look up rows based on columns other than the primary key. These indexes are stored separately from the data rows and contain a copy of the indexed column(s) along with a reference to the corresponding data row. Secondary indexes are used for efficient retrieval of data based on non-primary key columns.

Here’s a comparison of clustered and non-clustered index concepts in MySQL:

Primary Key Index (Implicitly Clustered):

• Determines the physical order of data rows in the table.
• Ensures uniqueness of rows.
• Used for quick retrieval of data based on the primary key column(s).

Secondary Index (Non-Clustered):

• Used for quick retrieval of data based on non-primary key column(s).
• Does not determine the physical order of data rows; data rows are physically ordered based on the primary key index.
• Secondary indexes store a copy of the indexed column(s) and a reference to the corresponding data row.

In summary, while MySQL does not use the terms “clustered” and “non-clustered” index, the concept of a clustered index is closely tied to the primary key index in MySQL’s InnoDB storage engine. Secondary indexes are used for non-primary key columns and do not determine the physical order of data rows.

The choice of primary key and the use of secondary indexes can significantly impact the performance and efficiency of your MySQL queries. Understanding how primary and secondary indexes work is essential for designing efficient database schemas.