What is the difference between clustered and non-clustered indexes in MySQL?

Clustered indexes and non-clustered indexes in MySQL are two different types of indexes that have some differences when it comes to storing and querying data.

1. Clustered index:

1. Clustered index stores the data rows in the leaf nodes of the index instead of separate data pages. This means that the index itself is a part of the data, allowing direct access to the data through the index without the need for additional search processes.
2. Clustered indexes can only have one, typically being the primary key index of a table. As the data rows are stored in the order of the primary key, clustered indexes can significantly enhance the performance of primary key queries.
3. If there is no primary key or unique index in a table, MySQL will automatically create a hidden clustered index to ensure data uniqueness.

1. Non-clustered index

1. Non-clustered indexes separate the index and data storage. The index stores references to the data rows (pointers to the data pages). When querying, one must first use the index to find the reference to the data row, and then use that reference to locate the actual data row.
2. A table can have multiple non-clustered indexes, which can be either regular or unique indexes. Non-clustered indexes are suitable for range queries because the data is stored separately and allows for quick access to the needed data rows.
3. Non-clustered indexes can improve query efficiency, but in cases of large data volume, they may increase additional I/O overhead as queries need to first access the index before accessing data pages.

In general, clustered indexes are suitable for primary key and range queries, improving performance and saving storage space. Non-clustered indexes, on the other hand, are better for non-unique value queries and can speed up the query process. In practice, the choice of index type should be based on specific business needs and query patterns.