Question: What is a clustered index in PostgreSQL?

Answer

Clustered indexes, as a specific term, are not formally supported in PostgreSQL like they are in some other database systems such as SQL Server or MySQL. In those systems, a clustered index determines the physical order of data within the table based on the key values in the index. However, PostgreSQL does have a related concept through the use of the CLUSTER command which can be used to reorder a table based on the index.

Understanding the CLUSTER Command

In PostgreSQL, the CLUSTER command is used to reorder the rows of a table more permanently according to the specified index. It physically rewrites the table row order to match the ordering of an index. This can improve the speed of data retrieval operations on tables by reducing the number of disk reads required for queries involving indexed columns.

How to Use CLUSTER

Here's how to use the CLUSTER command:

1. Create an Index: First, you must define an index on the table. For example:

   CREATE INDEX employee_idx ON employees (department_id);

2. Cluster the Table: Next, you can cluster the table. Once you execute this, PostgreSQL will reorder the table according to the specified index and future inserts into the table will not maintain this order.

   CLUSTER employees USING employee_idx;

After clustering, whenever the table is significantly updated (e.g., a large number of rows are inserted or deleted), it might be beneficial to re-cluster the table to maintain performance.

Limitations and Considerations

• Performance Cost: The CLUSTER operation can be costly in terms of time and resources, especially for large tables.
• Temporary Improvement: As new rows are added, the benefits of clustering can diminish over time unless the table is reclustered.
• Table Locking: The table is locked for write operations during the clustering process, which may not be acceptable in high-availability environments.

Alternatives

For scenarios where frequent updates occur, consider using a combination of regular indexing with additional performance optimization strategies like partitioning or using the VACUUM FULL command, which also compacts the table but without regard to any particular index.

In summary, while PostgreSQL does not support clustered indexes natively in the way that some other RDBMS do, you can achieve similar benefits by using the CLUSTER command along with careful planning regarding the operation's impact on your database environment.