Unused indexes in PostgreSQL refer to indexes that are created on tables but are not actively used. These indexes consume disk space, require maintenance, and can negatively affect the performance.
Here are a few reasons why you should care about unused indexes in Postgres:
Storage and disk space: Unused indexes occupy disk space that could be better utilized for other purposes. This can result in increased storage costs and reduce the available space for other database objects.
Performance impact: Indexes incur overhead during data modification operations, such as inserts, updates, and deletes. When there are many unused indexes, these operations take longer because the database must update multiple indexes in addition to the table.
Slower query execution: Postgres' query optimizer considers all available indexes when generating an execution plan for a query. If there are unused indexes, the optimizer may spend additional time considering these indexes, leading to suboptimal query plans and slower query execution.
Maintenance overhead: Maintaining indexes requires resources, including CPU and disk I/O. If you have a large number of unused indexes, these resources are wasted on unnecessary index maintenance tasks.
Note that you might have unused indexes on a primary machine but are used on a replica.