What are indexes used for in relational databases (PostgreSQL)? When do they help, and when do they hinder?

Indexes in relational databases such as PostgreSQL are used to speed up search and data retrieval operations. They create a structure (for example, a B-tree) that allows quick finding of rows based on key values without scanning the entire table.

Indexes help when:

• Queries with WHERE conditions on indexed columns are frequently executed.
• JOIN operations on indexed keys are performed.
• ORDER BY sorts on indexed fields are used.

However, indexes can be detrimental:

• During frequent insert, update, and delete operations, as the index needs to be updated, which slows down these operations.
• If an index is created on a column with high cardinality but queries do not use this index effectively.
• If the table is small — a full scan might be faster.

Example of creating an index in PostgreSQL:

CREATE INDEX idx_users_email ON users(email);

This index will speed up searching for users by email.