What are indexes in relational databases, when do they help, and when do they hinder?

Indexes in relational databases are data structures that speed up the search and retrieval of rows based on certain columns.

When they help:

• During frequent queries with WHERE conditions on indexed columns.
• During JOINs on indexed keys.
• When sorting with ORDER BY if the index covers the sorted fields.
• To ensure uniqueness (unique indexes).

When they hinder:

• During frequent insert, update, or delete operations, as indexes need to be updated, which reduces write performance.
• If the index is created on columns with low selectivity (few unique values), it may not be used or may even worsen the execution plan.
• With an excessive number of indexes — increasing storage and maintenance load.

In summary: indexes are a powerful tool to accelerate reading, but they require balancing considering the write load and query characteristics.