What are database views and materialized views?

Views are virtual tables defined by queries - they don't store data but execute the query each time accessed. Uses: simplify complex queries, provide security (expose subset of data), abstract schema changes. Materialized views store query results physically, updating periodically or on demand. They trade storage for query speed. Use regular views for security/abstraction, materialized views for expensive queries with acceptable staleness. PostgreSQL supports incremental refresh.